我有以下表达式，类型为表达式< Func< TDocument，object>>
x => x.Name现在，我不知道 x的类型。在编译时命名，但是现在它在运行时，因为它存储在一个类型中。
如何将我的表达式转换为类型表达式&FunC< TDocument，TOutput>> 其中 TOutput 是一个类型，在编译时不知道？
解决方案你只需要在转换表达式中包装原始表达式的 Body ，然后重建lambda。以下是我可以如何使用泛型：
表达式&FunC< TInput，TReturn> ;> ConvertReturnValue< TInput，TReturn>（表达式< Func< TInput，object>> inputExpression） {表达式convertedExpressionBody = Expression.Convert（ inputExpression.Body，typeof（TReturn ））; 返回Expression.Lambda无泛型
如果您不能使用泛型，因为您在编译时不知道返回类型， Expression.Lambda 实际上提供了非泛型重载，您可以使用它this：
Expression ConvertReturnValue< TInput>（Expression< Func< TInput，object>> inputExpression，类型returnType） { Expression convertedExpressionBody = Expression.Convert（inputExpression.Body，returnType）; 返回Expression.Lambda（convertedExpressionBody，inputExpression.Parameters）; }上述仍然返回一个表达式&FunC< TInput ，TReturn>> （upcast到非泛型 Expression ）。如果需要，您可以稍后下载：
表达式< Func< TDocument，object>> inputExpression = d => d.Name; 表达式< Func< TDocument，string>> convertExpression =（表达式& Func< TDocument，string>>）ConvertReturnValue（inputExpression，typeof（string））; //测试。 TDocument doc = new TDocument {Name =Zzz}; string name = convertedExpression.Compile（）。Invoke（doc）; Assert.Equal（Zzz，name）;附录
<请注意，对于struct返回类型，最终的表达式可能会如下所示： （TDocument d）=> （INT）（对象）d.ID;
I have the following expression which is of type Expression<Func<TDocument, object>>
x => x.NameNow, I don't know the type of x.Name at compile time, but I now it at runtime since it's stored in a Type.
How can I convert my expression to be of type Expression<Func<TDocument, TOutput>> where TOutput is a Type and not known at compile time?
解决方案You just need to wrap the original expression's Body in a Convert expression and then rebuild your lambda. Here's how I would do it if I could use generics:
Expression<Func<TInput, TReturn>> ConvertReturnValue<TInput, TReturn>( Expression<Func<TInput, object>> inputExpression) { Expression convertedExpressionBody = Expression.Convert( inputExpression.Body, typeof(TReturn) ); return Expression.Lambda<Func<TInput, TReturn>>( convertedExpressionBody, inputExpression.Parameters ); }Usage:
Expression<Func<TDocument, object>> inputExpression = d => d.Name; Expression<Func<TDocument, string>> convertedExpression = ConvertReturnValue<TDocument, string>(inputExpression); // Test. TDocument doc = new TDocument { Name = "Zzz" }; string name = convertedExpression.Compile().Invoke(doc); Assert.Equal("Zzz", name);No generics
If you cannot use generics because you don't know the return type at compile time, Expression.Lambda actually offers a non-generic overload, which you can use like this:
Expression ConvertReturnValue<TInput>(Expression<Func<TInput, object>> inputExpression, Type returnType) { Expression convertedExpressionBody = Expression.Convert(inputExpression.Body, returnType); return Expression.Lambda(convertedExpressionBody, inputExpression.Parameters); }The above still returns an Expression<Func<TInput, TReturn>> (upcast to a non-generic Expression). You can downcast it later if you need to:
Expression<Func<TDocument, object>> inputExpression = d => d.Name; Expression<Func<TDocument, string>> convertedExpression = (Expression<Func<TDocument, string>>)ConvertReturnValue(inputExpression, typeof(string)); // Test. TDocument doc = new TDocument { Name = "Zzz" }; string name = convertedExpression.Compile().Invoke(doc); Assert.Equal("Zzz", name);Addendum
Note that for struct return types, the final expression may end up looking like this:
(TDocument d) => (int)(object)d.ID;
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