Why does foreach fail to find my GetEnumerator extension method?

Question

I\'m trying to make some code more readable. For Example foreach(var row in table) {...} rather than foreach(DataRow row in table.Rows) {...}.

Answer 1

Your extension is equivalent to:

    public static IEnumerable<TDataRow> GetEnumerator<TDataRow>( this DataTable tbl ) {
        foreach ( TDataRow r in tbl.Rows ) yield return r;
    }

GetEnumerator<TDataRow> is not the same method as GetEnumerator

This will work better:

    public static IEnumerable<DataRow> GetEnumerator( this DataTable tbl ) {
        foreach (DataRow r in tbl.Rows ) yield return r;
    }

Answer 2

The GetEnumerator method in your test class is not static, the extension method is. This doesn't compile either:

class test
{
}

static class x
{
    public static IEnumerator<object> GetEnumerator(this test t) { return null; }
}

class Program
{
    static void Main(string[] args)
    {
        foreach (var i in new test()) {  }
    }
}

In order for the foreach syntax sugar to work your class must expose a public GetEnumerator instance method.

Edit:

As of C# 9.0, GetEnumerator can be an extension method.