Distinct for collection of objects of anonymus types.

internal sealed class DelegatingComparer<TSource, TField> : IEqualityComparer<TSource> where TField : class
{
    private readonly Func<TSource, TField> _getField;

    internal DelegatingComparer(Func<TSource, TField> getField)
    {
        if (null == getField)
        {
            throw new ArgumentNullException("getField");
        }

        _getField = getField;
    }

    public bool Equals(TSource x, TSource y)
    {
        var a = _getField(x);
        var b = _getField(y);

        if (null == a)
        {
            return null == b;
        }

        return a.Equals(b);
    }

    public int GetHashCode(TSource @object)
    {
        var fieldValue = _getField(@object);
        return null == fieldValue ? 0 : fieldValue.GetHashCode();
    }
}

internal static class DelegatingExtensions
{
    internal static IEnumerable<T> MyDistinct<T, TField>(this IEnumerable<T> source, Func<T, TField> getField) where TField : class
    {
        if (null == source)
        {
            throw new ArgumentNullException("source");
        }

        if (null == getField)
        {
            throw new ArgumentNullException("getField");
        }

        return source.Distinct(new DelegatingComparer<T, TField>(getField));
    }
}

//usage

var names = new [] { "Петя", "Вася", "Вася", "Коля", "Пахом" };
var original = names.Select(n => new { Name = n, n.Length }); // how to call Enumerable.Distinct?

var filtered = original.MyDistinct(v => v.Name);