Question

I am using Dynamic Linq to query our database with a string that is created based on the filters a user enters. In one case, we need to perform a coalesce on two fields in order to apply our filter to the result.

The resulting syntax would look something like this:

(nullableField ?? requiredField) == "foo"

Or, if using an extension method:

(nullableField ?? requiredField).Contains("foo")

Dynamic Linq, or at least to version we currently have in our project, doesn’t support coalesce, are there any implementations available or suggestions as to how I can implement my own? I have little experience with expressions and I’m struggling to create my own. So far, the following code works on my limited testing but I’m not sure if I have done it correctly.

This is called by ParseExpression():

    // ?? operator
    Expression ParseCoalesce()
    {
        Expression left = ParseLogicalOr();
        if (token.id == TokenId.DoubleQuestion)
        {
            NextToken();
            Expression expr1 = ParseExpression();
            left = Expression.Coalesce(left, expr1);
        }
        return left;
    }

Thoughts on this code? Is there a better way to do it?

Answer 1

If your system supports ConditionalExpression, you could implement it like

(nullableField != null ? nullableField == "foo" : requiredField == "foo")

or

(nullableField != null ? nullableField : requiredField) == "foo"