我有一个暴露一些异步方法的接口。更具体地说，它具有定义的方法，其返回任务或任务＆lt; T＆gt;。我使用的是async / await关键字。
我正在实施此界面。但是，在其中一些方法中，这个实现没有任何东西需要等待。出于这个原因，我收到编译器警告＆＃34;这种异步方法缺乏等待＆＃39;等待＆＃39;运营商并将同步运行......＆＃34;
我理解为什么我会收到错误，但我想知道在这种情况下我是否应该对它们采取任何措施。忽略编译器警告感觉不对。
我知道我可以通过等待Task.Run来修复它，但对于只进行一些廉价操作的方法来说感觉不对。它听起来也会为执行添加不必要的开销，但是我也不确定是否已经存在，因为存在async关键字。
我应该忽略这些警告，还是有办法解决这个问题，我没有看到？
答案 0 :(得分：91)
async 关键字只是方法的实现细节;它不是方法签名的一部分。如果某个特定方法实现或覆盖无需等待，则只需省略 async 关键字并使用Task.FromResult<TResult>返回已完成的任务：
public Task<string> Foo() // public async Task<string> Foo()
{ // {
Baz(); // Baz();
return Task.FromResult("Hello"); // return "Hello";
} // }
如果您的方法返回Task而不是Task<TResult>，那么您可以返回任何类型和值的已完成任务。 Task.FromResult(0)似乎是一个受欢迎的选择：
public Task Bar() // public async Task Bar()
{ // {
Baz(); // Baz();
return Task.FromResult(0); //
} // }
或者，从.NET Framework 4.6开始，您可以返回Task.CompletedTask：
public Task Bar() // public async Task Bar()
{ // {
Baz(); // Baz();
return Task.CompletedTask; //
} // }
答案 1 :(得分：9)
一些“异步”操作同步完成，但为了多态而仍然符合异步调用模型，这是完全合理的。
一个真实的例子是OS I / O API。某些设备上的异步和重叠调用始终完成内联（例如，写入使用共享内存实现的管道）。但它们实现了与多部分操作相同的界面，这些操作在后台继续进行。
答案 2 :(得分：2)
Michael Liu很好地回答了有关如何避免警告的问题：返回Task.FromResult。
我将回答您问题的“我是否应该担心警告”部分。
答案是肯定的！
其原因是，当您调用一个在没有Task运算符的异步方法内部返回await的方法时，经常会出现警告。我只是修复了一个并发性错误，该错误是因为我在不等待上一个操作的情况下在Entity Framework中调用了一个操作。
如果可以精心编写代码以避免编译器警告，那么当出现警告时，它会像拇指酸痛一样脱颖而出。我本可以避免几个小时的调试。
答案 3 :(得分：2)
仅当您实际调用所涉及的方法时，并且仅当性能是一个问题时。
这可以通过编写一个包含以下 4 个方法的程序，然后将它们反编译为 IL 来证明（注意，所呈现的 IL 可能因运行时版本而异；以下来自 .NET Core 3.1）：
int FullySync() => 42;
Task<int> TaskSync() => Task.FromResult(42);
// CS1998
async Task<int> NotActuallyAsync() => 42;
async Task<int> FullyAsync() => await Task.Run(() => 42);
前两个导致非常短的方法主体包含您所期望的内容：
.method private hidebysig
instance int32 FullySync () cil managed
{
// Method begins at RVA 0x205e
// Code size 3 (0x3)
.maxstack 8
// return 42;
IL_0000: ldc.i4.s 42
IL_0002: ret
} // end of method Program::FullySync
.method private hidebysig
instance class [System.Runtime]System.Threading.Tasks.Task`1<int32> TaskSync () cil managed
{
// Method begins at RVA 0x2062
// Code size 8 (0x8)
.maxstack 8
// return Task.FromResult(42);
IL_0000: ldc.i4.s 42
IL_0002: call class [System.Runtime]System.Threading.Tasks.Task`1<!!0> [System.Runtime]System.Threading.Tasks.Task::FromResult<int32>(!!0)
IL_0007: ret
} // end of method Program::TaskSync
但是最后两个关键字的 async 会导致编译器为这些方法生成 asynchronous state machines：
.method private hidebysig
instance class [System.Runtime]System.Threading.Tasks.Task`1<int32> NotActuallyAsync () cil managed
{
.custom instance void [System.Runtime]System.Runtime.CompilerServices.AsyncStateMachineAttribute::.ctor(class [System.Runtime]System.Type) = (
01 00 29 43 53 31 39 39 38 54 65 73 74 2e 50 72
6f 67 72 61 6d 2b 3c 4e 6f 74 41 63 74 75 61 6c
6c 79 41 73 79 6e 63 3e 64 5f 5f 33 00 00
)
.custom instance void [System.Diagnostics.Debug]System.Diagnostics.DebuggerStepThroughAttribute::.ctor() = (
01 00 00 00
)
// Method begins at RVA 0x206c
// Code size 56 (0x38)
.maxstack 2
.locals init (
[0] class CS1998Test.Program/'<NotActuallyAsync>d__3'
)
IL_0000: newobj instance void CS1998Test.Program/'<NotActuallyAsync>d__3'::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: call valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::Create()
IL_000c: stfld valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>t__builder'
IL_0011: ldloc.0
IL_0012: ldarg.0
IL_0013: stfld class CS1998Test.Program CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>4__this'
IL_0018: ldloc.0
IL_0019: ldc.i4.m1
IL_001a: stfld int32 CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>1__state'
IL_001f: ldloc.0
IL_0020: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>t__builder'
IL_0025: ldloca.s 0
IL_0027: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::Start<class CS1998Test.Program/'<NotActuallyAsync>d__3'>(!!0&)
IL_002c: ldloc.0
IL_002d: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>t__builder'
IL_0032: call instance class [System.Runtime]System.Threading.Tasks.Task`1<!0> valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::get_Task()
IL_0037: ret
} // end of method Program::NotActuallyAsync
.class nested private auto ansi sealed beforefieldinit '<NotActuallyAsync>d__3'
extends [System.Runtime]System.Object
implements [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine
{
.custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
01 00 00 00
)
// Fields
.field public int32 '<>1__state'
.field public valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> '<>t__builder'
.field public class CS1998Test.Program '<>4__this'
// Methods
.method public hidebysig specialname rtspecialname
instance void .ctor () cil managed
{
// Method begins at RVA 0x20fd
// Code size 8 (0x8)
.maxstack 8
// {
IL_0000: ldarg.0
// (no C# code)
IL_0001: call instance void [System.Runtime]System.Object::.ctor()
// }
IL_0006: nop
IL_0007: ret
} // end of method '<NotActuallyAsync>d__3'::.ctor
.method private final hidebysig newslot virtual
instance void MoveNext () cil managed
{
.override method instance void [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext()
// Method begins at RVA 0x2108
// Code size 58 (0x3a)
.maxstack 2
.locals init (
[0] int32,
[1] int32,
[2] class [System.Runtime]System.Exception
)
// int num = <>1__state;
IL_0000: ldarg.0
IL_0001: ldfld int32 CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>1__state'
IL_0006: stloc.0
.try
{
// result = 42;
IL_0007: ldc.i4.s 42
IL_0009: stloc.1
// }
IL_000a: leave.s IL_0024
} // end .try
catch [System.Runtime]System.Exception
{
// catch (Exception exception)
IL_000c: stloc.2
// <>1__state = -2;
IL_000d: ldarg.0
IL_000e: ldc.i4.s -2
IL_0010: stfld int32 CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>1__state'
// <>t__builder.SetException(exception);
IL_0015: ldarg.0
IL_0016: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>t__builder'
IL_001b: ldloc.2
IL_001c: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::SetException(class [System.Runtime]System.Exception)
// return;
IL_0021: nop
IL_0022: leave.s IL_0039
} // end handler
// <>1__state = -2;
IL_0024: ldarg.0
IL_0025: ldc.i4.s -2
IL_0027: stfld int32 CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>1__state'
// <>t__builder.SetResult(result);
IL_002c: ldarg.0
IL_002d: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<NotActuallyAsync>d__3'::'<>t__builder'
IL_0032: ldloc.1
IL_0033: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::SetResult(!0)
// }
IL_0038: nop
IL_0039: ret
} // end of method '<NotActuallyAsync>d__3'::MoveNext
.method private final hidebysig newslot virtual
instance void SetStateMachine (
class [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine stateMachine
) cil managed
{
.custom instance void [System.Diagnostics.Debug]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
01 00 00 00
)
.override method instance void [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine::SetStateMachine(class [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine)
// Method begins at RVA 0x2160
// Code size 1 (0x1)
.maxstack 8
// }
IL_0000: ret
} // end of method '<NotActuallyAsync>d__3'::SetStateMachine
} // end of class <NotActuallyAsync>d__3
.method private hidebysig
instance class [System.Runtime]System.Threading.Tasks.Task`1<int32> FullyAsync () cil managed
{
.custom instance void [System.Runtime]System.Runtime.CompilerServices.AsyncStateMachineAttribute::.ctor(class [System.Runtime]System.Type) = (
01 00 23 43 53 31 39 39 38 54 65 73 74 2e 50 72
6f 67 72 61 6d 2b 3c 46 75 6c 6c 79 41 73 79 6e
63 3e 64 5f 5f 34 00 00
)
.custom instance void [System.Diagnostics.Debug]System.Diagnostics.DebuggerStepThroughAttribute::.ctor() = (
01 00 00 00
)
// Method begins at RVA 0x20b0
// Code size 56 (0x38)
.maxstack 2
.locals init (
[0] class CS1998Test.Program/'<FullyAsync>d__4'
)
IL_0000: newobj instance void CS1998Test.Program/'<FullyAsync>d__4'::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: call valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::Create()
IL_000c: stfld valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>t__builder'
IL_0011: ldloc.0
IL_0012: ldarg.0
IL_0013: stfld class CS1998Test.Program CS1998Test.Program/'<FullyAsync>d__4'::'<>4__this'
IL_0018: ldloc.0
IL_0019: ldc.i4.m1
IL_001a: stfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>1__state'
IL_001f: ldloc.0
IL_0020: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>t__builder'
IL_0025: ldloca.s 0
IL_0027: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::Start<class CS1998Test.Program/'<FullyAsync>d__4'>(!!0&)
IL_002c: ldloc.0
IL_002d: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>t__builder'
IL_0032: call instance class [System.Runtime]System.Threading.Tasks.Task`1<!0> valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::get_Task()
IL_0037: ret
} // end of method Program::FullyAsync
.class nested private auto ansi sealed beforefieldinit '<FullyAsync>d__4'
extends [System.Runtime]System.Object
implements [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine
{
.custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
01 00 00 00
)
// Fields
.field public int32 '<>1__state'
.field public valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> '<>t__builder'
.field public class CS1998Test.Program '<>4__this'
.field private int32 '<>s__1'
.field private valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32> '<>u__1'
// Methods
.method public hidebysig specialname rtspecialname
instance void .ctor () cil managed
{
// Method begins at RVA 0x217b
// Code size 8 (0x8)
.maxstack 8
// {
IL_0000: ldarg.0
// (no C# code)
IL_0001: call instance void [System.Runtime]System.Object::.ctor()
// }
IL_0006: nop
IL_0007: ret
} // end of method '<FullyAsync>d__4'::.ctor
.method private final hidebysig newslot virtual
instance void MoveNext () cil managed
{
.override method instance void [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext()
// Method begins at RVA 0x2184
// Code size 199 (0xc7)
.maxstack 3
.locals init (
[0] int32,
[1] int32,
[2] valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32>,
[3] class CS1998Test.Program/'<FullyAsync>d__4',
[4] class [System.Runtime]System.Exception
)
// int num = <>1__state;
IL_0000: ldarg.0
IL_0001: ldfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>1__state'
IL_0006: stloc.0
.try
{
// if (num != 0)
IL_0007: ldloc.0
IL_0008: brfalse.s IL_000c
// (no C# code)
IL_000a: br.s IL_000e
// awaiter = Task.Run(() => 42).GetAwaiter();
IL_000c: br.s IL_0065
IL_000e: ldsfld class [System.Runtime]System.Func`1<int32> CS1998Test.Program/'<>c'::'<>9__4_0'
IL_0013: dup
IL_0014: brtrue.s IL_002d
// (no C# code)
IL_0016: pop
// if (!awaiter.IsCompleted)
IL_0017: ldsfld class CS1998Test.Program/'<>c' CS1998Test.Program/'<>c'::'<>9'
IL_001c: ldftn instance int32 CS1998Test.Program/'<>c'::'<FullyAsync>b__4_0'()
IL_0022: newobj instance void class [System.Runtime]System.Func`1<int32>::.ctor(object, native int)
IL_0027: dup
IL_0028: stsfld class [System.Runtime]System.Func`1<int32> CS1998Test.Program/'<>c'::'<>9__4_0'
IL_002d: call class [System.Runtime]System.Threading.Tasks.Task`1<!!0> [System.Runtime]System.Threading.Tasks.Task::Run<int32>(class [System.Runtime]System.Func`1<!!0>)
IL_0032: callvirt instance valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<!0> class [System.Runtime]System.Threading.Tasks.Task`1<int32>::GetAwaiter()
IL_0037: stloc.2
IL_0038: ldloca.s 2
IL_003a: call instance bool valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32>::get_IsCompleted()
IL_003f: brtrue.s IL_0081
// num = (<>1__state = 0);
IL_0041: ldarg.0
IL_0042: ldc.i4.0
IL_0043: dup
IL_0044: stloc.0
IL_0045: stfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>1__state'
// <>u__1 = awaiter;
IL_004a: ldarg.0
IL_004b: ldloc.2
IL_004c: stfld valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>u__1'
// <FullyAsync>d__4 stateMachine = this;
IL_0051: ldarg.0
IL_0052: stloc.3
// <>t__builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
IL_0053: ldarg.0
IL_0054: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>t__builder'
IL_0059: ldloca.s 2
IL_005b: ldloca.s 3
IL_005d: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::AwaitUnsafeOnCompleted<valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32>, class CS1998Test.Program/'<FullyAsync>d__4'>(!!0&, !!1&)
// return;
IL_0062: nop
IL_0063: leave.s IL_00c6
// awaiter = <>u__1;
IL_0065: ldarg.0
IL_0066: ldfld valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>u__1'
IL_006b: stloc.2
// <>u__1 = default(TaskAwaiter<int>);
IL_006c: ldarg.0
IL_006d: ldflda valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>u__1'
IL_0072: initobj valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32>
// num = (<>1__state = -1);
IL_0078: ldarg.0
IL_0079: ldc.i4.m1
IL_007a: dup
IL_007b: stloc.0
IL_007c: stfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>1__state'
// <>s__1 = awaiter.GetResult();
IL_0081: ldarg.0
IL_0082: ldloca.s 2
IL_0084: call instance !0 valuetype [System.Runtime]System.Runtime.CompilerServices.TaskAwaiter`1<int32>::GetResult()
IL_0089: stfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>s__1'
// result = <>s__1;
IL_008e: ldarg.0
IL_008f: ldfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>s__1'
IL_0094: stloc.1
// }
IL_0095: leave.s IL_00b1
} // end .try
catch [System.Runtime]System.Exception
{
// catch (Exception exception)
IL_0097: stloc.s 4
// <>1__state = -2;
IL_0099: ldarg.0
IL_009a: ldc.i4.s -2
IL_009c: stfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>1__state'
// <>t__builder.SetException(exception);
IL_00a1: ldarg.0
IL_00a2: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>t__builder'
IL_00a7: ldloc.s 4
IL_00a9: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::SetException(class [System.Runtime]System.Exception)
// return;
IL_00ae: nop
IL_00af: leave.s IL_00c6
} // end handler
// <>1__state = -2;
IL_00b1: ldarg.0
IL_00b2: ldc.i4.s -2
IL_00b4: stfld int32 CS1998Test.Program/'<FullyAsync>d__4'::'<>1__state'
// <>t__builder.SetResult(result);
IL_00b9: ldarg.0
IL_00ba: ldflda valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> CS1998Test.Program/'<FullyAsync>d__4'::'<>t__builder'
IL_00bf: ldloc.1
IL_00c0: call instance void valuetype [System.Threading.Tasks]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::SetResult(!0)
// }
IL_00c5: nop
IL_00c6: ret
} // end of method '<FullyAsync>d__4'::MoveNext
.method private final hidebysig newslot virtual
instance void SetStateMachine (
class [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine stateMachine
) cil managed
{
.custom instance void [System.Diagnostics.Debug]System.Diagnostics.DebuggerHiddenAttribute::.ctor() = (
01 00 00 00
)
.override method instance void [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine::SetStateMachine(class [System.Runtime]System.Runtime.CompilerServices.IAsyncStateMachine)
// Method begins at RVA 0x2268
// Code size 1 (0x1)
.maxstack 8
// }
IL_0000: ret
} // end of method '<FullyAsync>d__4'::SetStateMachine
} // end of class <FullyAsync>d__4
简而言之，执行标有 async 修饰符的方法需要为该方法构建和执行异步状态机，无论该方法是否实际执行任何异步工作 ！我相信您可以猜到，与标准的非异步方法相比，这会带来性能损失，这取决于您的用例，可能会也可能不会很重要。
但这根本不是 CS1998 警告所说的。此警告适用于以下情况：您定义了 async 方法，因为您需要等待其中的某些内容，但只是忘记在异步调用之前添加 await 关键字。
您的情况基本上相反：您已将方法定义为 async，但您知道并打算它不执行任何此类工作。但是编译器无法知道这一点 - 对编译器来说，它看起来与前一种情况完全相同，因此您会收到相同的警告。
老实说，在第二种情况下您自己在实现中不必要地添加了 async 关键字而导致警告。您知道该方法不执行任何异步工作，那么为什么还要添加关键字呢？你只是无缘无故地膨胀。
当然可以改进警告，指出您基本上是愚蠢的，and I've opened an issue in the Roslyn repo to hopefully get that done。
答案 4 :(得分：1)
我发现了一个技巧来绕过此警告：
public async Task<object> test()
{
//a pseudo code just to disable the warning about lack of await in async code!
var xyz = true ? 0 : await Task.FromResult(0); //use a var name that's not used later
//... your code statements as normal, eg:
//throw new NotImplementedException();
}
该 await 关键字的存在会欺骗编译器不发出警告，即使我们知道它永远不会被调用！因为条件是true，所以它总是返回三进制条件（？:)的第一部分，并且由于此var是不使用的var，因此在Release版本中将省略它。我不确定这种方法是否有副作用。
答案 5 :(得分：0)
Task.FromResult时的异常行为的说明这里有一个小样的演示，显示了标记和未标记async的方法在异常处理方面的区别。
public Task<string> GetToken1WithoutAsync() => throw new Exception("Ex1!");
// Warning: This async method lacks 'await' operators and will run synchronously. Consider ...
public async Task<string> GetToken2WithAsync() => throw new Exception("Ex2!");
public string GetToken3Throws() => throw new Exception("Ex3!");
public async Task<string> GetToken3WithAsync() => await Task.Run(GetToken3Throws);
public async Task<string> GetToken4WithAsync() { throw new Exception("Ex4!"); return await Task.FromResult("X");}
public static async Task Main(string[] args)
{
var p = new Program();
try { var task1 = p.GetToken1WithoutAsync(); }
catch( Exception ) { Console.WriteLine("Throws before await.");};
var task2 = p.GetToken2WithAsync(); // Does not throw;
try { var token2 = await task2; }
catch( Exception ) { Console.WriteLine("Throws on await.");};
var task3 = p.GetToken3WithAsync(); // Does not throw;
try { var token3 = await task3; }
catch( Exception ) { Console.WriteLine("Throws on await.");};
var task4 = p.GetToken4WithAsync(); // Does not throw;
try { var token4 = await task4; }
catch( Exception ) { Console.WriteLine("Throws on await.");};
}
// .NETCoreApp,Version=v3.0
Throws before await.
Throws on await.
Throws on await.
Throws on await.
（我对When async Task<T> required by interface, how to get return variable without compiler warning的回答的开头）
答案 6 :(得分：-1)
可能为时已晚，但可能对调查很有帮助：
关于已编译代码（ IL ）的内部结构：
public static async Task<int> GetTestData()
{
return 12;
}
在IL中变为：
.method private hidebysig static class [mscorlib]System.Threading.Tasks.Task`1<int32>
GetTestData() cil managed
{
.custom instance void [mscorlib]System.Runtime.CompilerServices.AsyncStateMachineAttribute::.ctor(class [mscorlib]System.Type) = ( 01 00 28 55 73 61 67 65 4C 69 62 72 61 72 79 2E // ..(UsageLibrary.
53 74 61 72 74 54 79 70 65 2B 3C 47 65 74 54 65 // StartType+<GetTe
73 74 44 61 74 61 3E 64 5F 5F 31 00 00 ) // stData>d__1..
.custom instance void [mscorlib]System.Diagnostics.DebuggerStepThroughAttribute::.ctor() = ( 01 00 00 00 )
// Code size 52 (0x34)
.maxstack 2
.locals init ([0] class UsageLibrary.StartType/'<GetTestData>d__1' V_0,
[1] valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> V_1)
IL_0000: newobj instance void UsageLibrary.StartType/'<GetTestData>d__1'::.ctor()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: call valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::Create()
IL_000c: stfld valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> UsageLibrary.StartType/'<GetTestData>d__1'::'<>t__builder'
IL_0011: ldloc.0
IL_0012: ldc.i4.m1
IL_0013: stfld int32 UsageLibrary.StartType/'<GetTestData>d__1'::'<>1__state'
IL_0018: ldloc.0
IL_0019: ldfld valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> UsageLibrary.StartType/'<GetTestData>d__1'::'<>t__builder'
IL_001e: stloc.1
IL_001f: ldloca.s V_1
IL_0021: ldloca.s V_0
IL_0023: call instance void valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::Start<class UsageLibrary.StartType/'<GetTestData>d__1'>(!!0&)
IL_0028: ldloc.0
IL_0029: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32> UsageLibrary.StartType/'<GetTestData>d__1'::'<>t__builder'
IL_002e: call instance class [mscorlib]System.Threading.Tasks.Task`1<!0> valuetype [mscorlib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<int32>::get_Task()
IL_0033: ret
} // end of method StartType::GetTestData
并且没有异步和任务方法：
public static int GetTestData()
{
return 12;
}
成为：
.method private hidebysig static int32 GetTestData() cil managed
{
// Code size 8 (0x8)
.maxstack 1
.locals init ([0] int32 V_0)
IL_0000: nop
IL_0001: ldc.i4.s 12
IL_0003: stloc.0
IL_0004: br.s IL_0006
IL_0006: ldloc.0
IL_0007: ret
} // end of method StartType::GetTestData
您会看到这些方法之间的巨大差异。如果您不在异步方法中使用await并且不关心使用异步方法（例如API调用或事件处理程序），那么好的主意会将其转换为普通的同步方法（这样可以节省应用程序的性能）。