Language Support for Combining Expressions?

[TonyValenti]

Maybe I'm missing something but it seems like it should be really easy to combine expressions or use them as sub expressions.

How would I do this:

public task RemapUsers(Expression<Func<User, string>> expression){
  var Data = DbContext.Set<User>().Where(x => expression(x) == "Hello").ToLiust();
}

[jnm2]

I haven't tried this, but it seems like it could work:

public Task RemapUsers(Expression<Func<User, string>> expression)
{
    var predicate = Expression.Lambda<Func<User, bool>>(
        Expression.Equal(expression.Body, Expression.Constant("Hello")),
        expression.Parameters);

    var Data = DbContext.Set<User>().Where(predicate).ToList();
}

The general case of inlining an expression like this seems like it involves creating a new tree by rewriting the expression.Body to replace the expression.Parameters and building up the expression around it using the Expression factory API.

[TonyValenti]

Any chance the language could do all this for me? This seems extremely complex.

[jnm2]

Sure seems handy occasionally. Maybe a complex proposal to give invocation expressions a new kind of meaning within expression trees when the receiver type is a lambda Expression instance.

The compiler would probably want a new Expression API to perform the inlining, probably, since it must happen at runtime. Unless the Expression API itself already supports expressing a call to an independently created lambda Expression instance.

[HaloFour]

I've had many cases when working with EF where I would have liked to have been able to write a snippet of expression tree that could be reused in other expression trees. I did have a few helper functions which could combine expression trees or visit/replace method invocation expressions with a constant value or a chunk of expression tree, but it was awkward to use.

[canton7]

The general case of inlining an expression like this seems like it involves creating a new tree by rewriting the expression.Body to replace the expression.Parameters

That's exactly it, see e.g. https://stackoverflow.com/a/58630886/1086121 for a general solution.

[svick]

You can use LINQKit for this:

var Data = DbContext.Set<User>().AsExpandable().Where(x => expression.Invoke(x) == "Hello").ToList();

Considering that LINQKit exists and that this is a niche use case, I don't think it makes sense to add language support for this.