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Compiler Error CS1939

Visual Studio 2008

Cannot pass the range variable 'name' as an out or ref parameter.

A range variable is a read-only variable that is introduced in a query expression and serves as an identifier for each successive element in a source sequence. Because it cannot be modified in any way, there is no point in passing it by ref or out. Therefore, both operations are not valid.

To correct this error

  • Pass the range variable by value.

The following example generates CS1939:

// cs1939.cs
using System.Linq;
class Test
    public static void F(ref int i)
    public static void Main()
        var list = new int[] { 0, 1, 2, 3, 4, 5 };
        var q = from x in list
                let k = x
                select Test.F(ref x); // CS1939