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Compiler Error CS1940

Visual Studio 2008

Multiple implementations of the query pattern were found for source type 'type'. Ambiguous call to 'method'.

This error is generated when multiple implementations of a query method are defined and the compiler cannot disambiguate which one is best to use for the query. In the following example, both versions of Select have the same signature, because they both accept one int as an input parameter and have int as a return value.

To correct this error

  • Provide only one implementation for each method.

The following code generates CS1940:

// cs1940.cs
using System; //must include explicitly for types defined in 3.5
class Test
    public delegate int Dele(int x);
    int num = 0;
    public int Select(Func<int, int> d)
        return d(this.num);
    public int Select(Dele d) // CS1940
        return d(this.num) + 1;
    public static void Main()
        var q = from x in new Test()
        select x;