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I have the following datatable, which has Units information. The Units have a surface in square meters. However the units can be split into subunits, in the datatable there is a ParentUnitID to know if that unit is splitted or not.

I need to validate that the sum of the surface of the child units, is equal to the surface of the parent unit.


In anoter forum they helped me with this code

 var childrenByParentID = _unitDataSet.Unit.AsEnumerable().
                                     GroupBy(child => child.Field<int?>("ParentUnitId")).
                                     ToDictionary(group => group.Key);

            // Parents = children with no parent
            var parents = childrenByParentID[null];

            // Look for the first group which sum of surfaces is not the same as the parent's surface
            var invalidGroup = childrenByParentID.FirstOrDefault(group =>
                bool invalid = false;

                if (group.Key != null)
                    // Not the parents group
                    var parent = parents.Single(parent => parent.Field<int>("UnitID") == group.Key);

                    var totalSurface = group.Sum(group => group.Field<int>("Surface"));
                    invalid = (totalSurface != parent.Field<int>("Surface"));

                return invalid;

            if (invalidGroup != null)
                // .. do something special

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However I got this error.

1 Solution
There're some common mistakes in this code. You've to change it->

  var parent = parents.Single(parent => parent.Field<int>("UnitID") == group.Key);
  var parent = parents.Single(p => p.Field<int>("UnitID") == group.Key);

    var totalSurface = group.Sum(group => group.Field<int>("Surface"));
    var totalSurface = group.Sum(g => g.Field<int>("Surface"));

last thing, you cannot compare object of KeyValuePair<,> with null, because its a Structure (valuetype) returned by following code
   invalidGroup = childrenByParentID.FirstOrDefault ......

try using ToLookup instead of ToDictionary

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