Question

I’m building a LINQ query dynamically based on user input, and I want to handle the case where the user is searching for a record r based on a string str and field foo where str.Contains(r.foo). Now, the reverse (r.foo.Contains(str)) is simple, but LINQ is giving me grief about doing it the other way.

Here’s what I have so far:

private Expression SqlNotIn(Expression left, Expression right)
{
    return Expression.Equal(
        Expression.Call(
            null,
            typeof(SqlFunctions).GetMethod("CharIndex", BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(string), typeof(string) }, null),
            new[] { right, left }
        ),
        Expression.Constant(0)
    );
}

This is supposed to take an Expression left, which is the property accessor, and Expression right, which is a constant of the string to search in, and return an Expression representing (essentially) (SqlFunctions.CharIndex(right, left) == 0). When I run this, I get “The binary operator Equal is not defined for the types ‘System.Nullable1[System.Int32]' and 'System.Int32'." Explicitly casting the0toint?` with an as expression seemed to cause LINQ to run the query early.

Is there a simple way to do this?

---

EDIT:

private Expression SqlNotIn(Expression left, Expression right)
{
    return Expression.Equal(
        Expression.Call(
            right,
            typeof(string).GetMethod("IndexOf", new[] { typeof(string) }),
            new[] { left }
        ),
        Expression.Constant(-1)
    );
}

This works, but the SQL it generates looks like this:

(CASE 
    WHEN DATALENGTH([t0].[Destination]) = 0 THEN 0
    ELSE CHARINDEX([t0].[Destination], @p0) - 1
 END) = @p1

I’d be happy to use CharIndex if I could.

Answer 1

You can try to convert first part to int, or second part to int? with Expression.Convert http://msdn.microsoft.com/en-us/library/bb292051.aspx

For example. Untested.

private Expression SqlNotIn(Expression left, Expression right) {
            return Expression.Equal(
                Expression.Convert(Expression.Call(
                    null,
                    typeof(SqlFunctions).GetMethod("CharIndex", BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(string), typeof(string) }, null),
                    new[] { right, left }
                ), typeof(int)),
                Expression.Constant(-1)
            );
        }

By the way you could use a string inputValue as second parameter, and use it with Expression.Constant(inputValue)

EDIT :

public static Expression SqlNotIn(Expression left, string right) {
            var method = typeof(string).GetMethod("IndexOf",
                new[] { typeof(string)});

            var call = Expression.Call(Expression.Constant(right), method, new []{left});

            var result =  Expression.Equal(call, Expression.Constant(0));
            return result;
        }

EDIT 2:

private Expression SqlNotIn2(Expression left, Expression right) {
            return Expression.Equal(
                Expression.Call(
                    null,
                    typeof(SqlFunctions).GetMethod("PatIndex", BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(string), typeof(string) }, null),
                    new[] { right, left }
                ),
                Expression.Convert(Expression.Constant(0), typeof(int ?))
            );
        }