Question

我们想用表达式动态创建以下代码：

TestDatabaseEntities entities = new TestDatabaseEntities();
entities.Database.Log = s => System.Diagnostics.Debug.WriteLine(s); // to show generated tsql
// We wanna create the following line dynamically
List<Employee> employees = entities.Employees.Where(c => SqlFunctions.StringConvert((double?)c.SomeNumber).Trim().Contains("123")).ToList();

以上代码效果很好。现在我们想用动态表达它。
我们这样做了：

Type entityType = typeof(TEntity);
ParameterExpression parameterExpression = Expression.Parameter(entityType, "entity");

Type nullableDoubleType = typeof (double?);
//leftSile == {entity.SomeNumber}, it's OK
UnaryExpression unaryExpression  = Expression.Convert(leftSile, nullableDoubleType);
MethodInfo stringConvertMethodInfo = typeof(SqlFunctions).GetMethod("StringConvert", new[] { nullableDoubleType });
MethodInfo trimMethodInfo = typeof(string).GetMethod("Trim", new Type[] {});
ConstantExpression newRightSide = Expression.Constant(valueObject.ToString());

var traceVariable1 = Expression.Call(stringConvertMethodInfo, unaryExpression);
var traceVariable2 = Expression.Call(traceVariable1, trimMethodInfo);
Expression resultExpression = Expression.Call(traceVariable2, methodInfo, newRightSide);
Expression<Func<Employee, bool>> expression = Expression.Lambda<Func<TEntity, bool>>(resultExpression, parameterExpression);

// expression == {entity => (StringConvert(Convert(entity.SomeNumber)).Trim().Contains("123"))}

表达式已成功创建，但是当我们在EntityFramework中使用它时会引发异常：

List<Employee> employees2 = entities.Employees.Where(expression.Compile()).ToList();
//Exception :
//An unhandled exception of type 'System.NotSupportedException' occurred in EntityFramework.SqlServer.dll
//Additional information: This function can only be invoked from LINQ to Entities.

Answer 1

请在下面找到链接

Dynamic predicates for Linq-to-Entity queries

Dynamic Querying with LINQ-to-Entities and Expressions

Answer 2

我可以找到解决方案：

我们不必自己使用Compile()。我们想使用以下代码：

List<Employee> employees2 = entities.Employees.Where(expression).ToList();

在Linq中创建和调用SqlFunction到实体C＃？

2 个答案: