How to create a static lambda for use with expression building?

Question

As of C# 6, lambdas now default to instance methods, and will never be static (which I assume means they always capture now, which I guess is more efficient [seems to be faster given the discussions]).

See here: Why has a lambda with no capture changed from a static in C# 5 to an instance method in C# 6?

and here: Difference in CSC and Roslyn compiler's static lambda expression evaluation?

This causes issues now with using lambdas when creating static MethodInfos for calls to expression methods such as Expression.Convert(Expression, typeof({SomeType}), conversionMethodInfo);

So then, what is the new way of doing this? I tried to use the "static" modifier with lambdas and it doesn't work. For those who can't imagine such code, this might be one example:

Func <T1,T2> converter = static v => ConvertT1ToT2(v); // ('T' is whatever type you want)
Expression.Convert(expression, typeof({SomeType}), converter.Method) // (error: converter.Method.IsStatic is false)

Yes, obviously it doesn't work.

Answer 1

So then, what is the new way of doing this?

There isn't one. The spec never promised anything about the implementation details of lambda expressions. Which is why you should not depend on them. It's also why What's new in C# 6 does not mention this.

Assuming you need to use Expression.Convert with a custom MethodInfo, then you should promote the lambda into a static method:

private static T2 Converter(T1 v)
{
    return ConvertT1ToT2(v);
}

…

MethodInfo converter =
    typeof(ThisType).GetMethod("Converter", BindingFlags.NonPublic | BindingFlags.Static);
// OR:
MethodInfo converter = ((Func<T1, T2>)Converter).Method;

Expression.Convert(expression, typeof(SomeType), converter)

This way, you're not using a lambda, so it's guaranteed the MethodInfo refers to a static method.

Answer 2

In case others wish to know, in the end, I had to promote (demote? lol) my expressions to "Expression-bodied function members" instead, like this:

// (class method)
static string _Convert(object obj) => (obj as SomeType)?.SomeProperty ?? ReturnSomethingElse;

then in my method body:

Func<object, string> conversionDelegate = _Convert;
Expression exp = Expression.Convert(expression, typeof(SomeType), conversionDelegate.Method);

Edit: Some talk about non-capturing/static lambdas here: https://github.com/dotnet/csharplang/issues/275