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Compiler Error CS1943

An expression of type 'type' is not allowed in a subsequent from clause in a query expression with source type 'type'. Type inference failed in the call to 'method'.

All range variables must represent queryable types.

To correct this error

  1. Make sure that the type is a queryable type that implements IEnumerable, IEnumerable<T> or a derived interface, or any other type which has a query pattern defined for it.

  2. If the type is a non-generic IEnumerable, provide an explicit type on the range variable.

The following code generates CS1943:

// cs1943.cs
using System.Linq;
class Test
{
    class TestClass
    { }
    static void Main()
    {
        int[] nums = { 0, 1, 2, 3, 4, 5 };
        TestClass tc = new TestClass();
        
        var x = from n in nums
                from s in tc // CS1943
                select n + s;
    }
}

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