In C#, why can't an anonymous method contain a yield statement?

Question

I thought it would be nice to do something like this (with the lambda doing a yield return):

public IList Find(Expression

        

Answer 1

Maybe its just a syntax limitation. In Visual Basic .NET, which is very similar to C#, it is perfectly possible while awkward to write

Sub Main()
    Console.Write("x: ")
    Dim x = CInt(Console.ReadLine())
    For Each elem In Iterator Function()
                         Dim i = x
                         Do
                             Yield i
                             i += 1
                             x -= 1
                         Loop Until i = x + 20
                     End Function()
        Console.WriteLine($"{elem} to {x}")
    Next
    Console.ReadKey()
End Sub

Also note the parentheses ' here; the lambda function Iterator Function...End Function returns an IEnumerable(Of Integer) but is not such an object itself. It must be called to get that object.

The converted code by [1] raises errors in C# 7.3 (CS0149):

static void Main()
{
    Console.Write("x: ");
    var x = System.Convert.ToInt32(Console.ReadLine());
    // ERROR: CS0149 - Method name expected 
    foreach (var elem in () =>
    {
        var i = x;
        do
        {
            yield return i;
            i += 1;
            x -= 1;
        }
        while (!i == x + 20);
    }())
        Console.WriteLine($"{elem} to {x}");
    Console.ReadKey();
}

I strongly disagree to the reason given in the other answers that it's difficult for the compiler to handle. The Iterator Function() you see in the VB.NET example is specifically created for lambda iterators.

In VB, there is the Iterator keyword; it has no C# counterpart. IMHO, there is no real reason this is not a feature of C#.

So if you really, really want anonymous iterator functions, currently use Visual Basic or (I haven't checked it) F#, as stated in a comment of Part #7 in @Thomas Levesque's answer (do Ctrl+F for F#).