LINQ to SQL translation to SQL of custom method

Question

Is there a way to translate an expression to SQL to use with LINQ to SQL?

For example I have a method that compares two values.

Example:

MyComparer.Compare(value1, value2, ">") return value1 > value2
MyComparer.Compare(value1, value2, "=") return value1 == value2
MyComparer.Compare(value1, value2, "<=") return value1 <= value2

And I would like a query like:

var list = from i in dataContext.items
           where MyComparer.Compare(i.value, someValue, "some operator")
           select ...

This won't work because, obviously, MyComparer doesn't translate to SQL.

Maybe this is a twisted question, but how can I translate this method to SQL or is this possible?

Answer 1

If you are using .NET 4: "How to: Call Custom Database Functions (LINQ to Entities)".

Answer 2

The most pragmatic option may be compositon:

// build the core query
var list = from i in dataContext.items
           // most of your query
           select ...

// compose the value-filter if required
switch(someOperator) {
    case "=": list = list.Where(row => row.value1 == row.value2); break;
    case ">": list = list.Where(row => row.value1 > row.value2); break;
    case "<": list = list.Where(row => row.value1 < row.value2); break;
}

// now use "list"

If you need something re-usable, you're going to have to get your hands dirty building (and probably parsing) expression-trees (System.Linq.Expression). Not trivial.