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When you use group by Linq creates a new collection of items so you have two collections of items. Here's a solution to both problems: <ol> <li>summing any amount of members in one iteration and</li> <li>avoid duplicating your item's collection</li> </ol> Code: <pre><code>public static class LinqExtensions { /// <summary> /// Computes the sum of the sequence of System.Double values that are obtained /// by invoking one or more transform functions on each element of the input sequence. /// </summary> /// <param name="source">A sequence of values that are used to calculate a sum.</param> /// <param name="selectors">The transform functions to apply to each element.</param> public static double[] SumMany<TSource>(this IEnumerable<TSource> source, params Func<TSource, double>[] selectors) { if (selectors.Length == 0) { return null; } else { double[] result = new double[selectors.Length]; foreach (var item in source) { for (int i = 0; i < selectors.Length; i++) { result[i] += selectors[i](item); } } return result; } } /// <summary> /// Computes the sum of the sequence of System.Decimal values that are obtained /// by invoking one or more transform functions on each element of the input sequence. /// </summary> /// <param name="source">A sequence of values that are used to calculate a sum.</param> /// <param name="selectors">The transform functions to apply to each element.</param> public static double?[] SumMany<TSource>(this IEnumerable<TSource> source, params Func<TSource, double?>[] selectors) { if (selectors.Length == 0) { return null; } else { double?[] result = new double?[selectors.Length]; for (int i = 0; i < selectors.Length; i++) { result[i] = 0; } foreach (var item in source) { for (int i = 0; i < selectors.Length; i++) { double? value = selectors[i](item); if (value != null) { result[i] += value; } } } return result; } } } </code></pre> Here's the way you have to do the summation: <pre><code>double[] result = m.Items.SumMany(p => p.Total, q => q.Done); </code></pre> Here's a general example: <pre><code>struct MyStruct { public double x; public double y; } MyStruct[] ms = new MyStruct[2]; ms[0] = new MyStruct() { x = 3, y = 5 }; ms[1] = new MyStruct() { x = 4, y = 6 }; // sum both x and y members in one iteration without duplicating the array "ms" by GROUPing it double[] result = ms.SumMany(a => a.x, b => b.y); </code></pre> as you can see <pre><code>result[0] = 7 result[1] = 11 </code></pre>
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