LinqKit System.InvalidCastException在成员属性上调用方法提供的表达式时

Question

给定一个简单的父/子类结构.我想使用linqkit在父级上应用子lambda表达式.我还希望Lambda表达式由实用程序方法提供.

public class Foo
{
    public Bar Bar { get; set; }
}

public class Bar
{
    public string Value { get; set; }
    public static Expression<Func<Bar, bool>> GetLambdaX()
    {
        return c => c.Value == "A";
    }
}
...

Expression<Func<Foo, bool>> lx = c => Bar.GetLambdaX().Invoke(c.Bar);
Console.WriteLine(lx.Expand());

上面的代码抛出

System.InvalidCastException: Unable to cast object of type 
'System.Linq.Expressions.MethodCallExpression' to type 
'System.Linq.Expressions.LambdaExpression'.
at LinqKit.ExpressionExpander.VisitMethodCall(MethodCallExpression m)
at LinqKit.ExpressionVisitor.Visit(Expression exp)
at LinqKit.ExpressionVisitor.VisitLambda(LambdaExpression lambda)
at LinqKit.ExpressionVisitor.Visit(Expression exp)
at LinqKit.Extensions.Expand<TDelegate>(Expression`1 expr)

Answer 1

var lambdaX = Bar.GetLambdaX();

Expression<Func<Foo, bool>> lx = c => lambdaX.Invoke(c.Bar);

这有效.

写作要小心

Expression<Func<Foo, bool>> lx = ...

在赋值之前,编译器处理此行,并在您的情况下创建一个分配给lx的方法调用表达式(即用于调用Bar.GetLambdaX()).

如果你使用var ...那么Bar.GetLambdaX(); 调用并赋值,其值(即lambda表达式)稍后用于lx = ...