Question

我想创建一个参数（ParameterExpression类的实例），它应该实现多个接口（让它们是IComparable，IFormattable），如下所示：

//TypeWithIComparableIFormattable composite_type = ...;
ParameterExpression parameter = Parameter(composite_type, "composite_param");
// Usage specific methods of composite param from implemented interfaces

Answer 1

尽管变量/参数可以是不同类型的实例，但您不能像这样对它进行delcare：

IComparable, IFormattable obj;

要进行多界面实施检查，您可以：

Delcare您自己的界面，说IComparableAndFormattable，继承自IComparable和IFormattable。但是这种方法要求参数类型必须实现接口。
执行运行时检查。它很罗嗦，但保持代码保持灵活性：

使用Expression.TypeAs()方法将参数转换为所需类型。

var param = Expression.Parameter(typeof(object), "o");
// IComparable comparable;
var comparableDeclare = Expression.Parameter(typeof(IComparable), "comparable");
// comparable = o as IComparable;
var comparableAssign = Expression.Assign(comparableDeclare, Expression.TypeAs(param, typeof(IComparable)));
// if (comparable == (IComparable)null) 
// {
//    throw new ArgumentException("The parameter must be a instance of IComparable.", nameof(o));
// }
var comparableCheck = Expression.IfThen(Expression.Equal(comparableDeclare, Expression.Constant(null, typeof(IComparable))),
     ThrowNotTypeOf(typeof(IComparable), param.Name));

var formattableDeclare = Expression.Parameter(typeof(IFormattable), "formattable");
// formattable = o as IFormattable;
var formattableAssign = Expression.Assign(formattableDeclare, Expression.TypeAs(param, typeof(IFormattable)));
// if (formattable == (IFormattable)null) 
// {
//    throw new ArgumentException("The parameter must be a instance of IFormattable.", nameof(o));
// }
var formattableCheck = Expression.IfThen(
    Expression.Equal(formattableDeclare, Expression.Constant(null, typeof(IFormattable))),
    ThrowNotTypeOf(typeof(IFormattable), param.Name));

var block = Expression.Block(
    new [] {
        comparableDeclare, formattableDeclare
    }, // local variables
    comparableAssign, comparableCheck, formattableAssign, formattableCheck);

foreach (var exp in block.Expressions)
{
    Console.WriteLine(exp);
}


// Compile the expression tree
var method = Expression.Lambda<Action<object>>(block, param).Compile();

method.Invoke(new ComparableFormattable());

其中ThrowNotTypeOf是生成throw new ArgumentException语句的辅助方法：

private static Expression ThrowNotTypeOf(Type type, string paramName)
{
    var ctor = typeof(ArgumentException).GetConstructor(new[] { typeof(string), typeof(string) });
    Debug.Assert(ctor != null);

    var messageArg = Expression.Constant($"The parameter must be an instance of '{type.Name}'.");
    var paramArg = Expression.Constant(paramName);

    return Expression.Throw(Expression.New(ctor, messageArg, paramArg));
}

如何创建应该在表达式树中实现多个接口的参数？

1 个答案: