Partially allow 'is' pattern in expression tree

[NN---]

Currently 'is' pattern is not allowed in expression meaning you cannot replace == null with is null.
I suggest to add support for null checks since they can be mapped to == for reference types.

[HaloFour]

I'm not sure what you mean. is isn't a pattern, it's an operator that can compare an operand against a pattern. It can be used anywhere that expects a bool expression. The null pattern can be used anywhere that expects a pattern, including within a recursive pattern.

Can you provide an example of where you're trying to use is or null?

[theunrepentantgeek]

There's a semantic difference between is null and == null that can't be handwaved away.

Reference types can override == and do something custom for the == null case, but is null is specified as always doing that exact check and can't be overridden.

At a higher level, a much more important issue is that the expression trees were designed to carry the full semantic load of the expression through to the various query providers (LINQ for SQL, Entity Framework, etc) so they could use all of that information when generating the appropriate query.

Adding partial support for newer features by lowering those to existing structures (essentially, throwing out some of the semantic information) makes it impossible to update those query providers as the key information they need is being thrown away.

This is a classic example of a short term dirty trick that'll turn around and cause significant increased pain down the track.

[NN---]

I understand that there is a difference.
However you can use ?? in expression which is semantically exactly the same as is not null.

using System;
using System.Linq.Expressions;

public class C {
    public void M() {
        Expression<Func<string,string>> f = x => x != null ? x : "a";
        Expression<Func<string,string>> g = x => x ?? "a";
        
        // Expression<Func<string,string>> h = x => x is notnull ? x : "a";
    }
}

Sample

[HaloFour]

Ah, you meant expression trees specifically. No patterns are currently supported in expression trees. There is a discussion about expanding the types of syntax allowed in expression trees here: #158

[NN---]

Sorry, for misunderstanding.
Upvoted. Thanks.

[huoyaoyuan]

Expression tree is more about what raw code you wrote, not waht they will lower to.