Question

In our application we have number of AggregatorTypes (idea copied from SQL Server)
i.e. MinAggregator MaxAggregator, CountAggregator, And SumAggregator.

Now I should check if Aggregator supports type of provided values. For Count it is not important. For Max and Min I use checking for IComparable.
But what to use for SUM?
I tried Converter but it also supports string.

Why there is no ISummable 🙂

Also IsNumeric like checking is not a solution for me. For example TimeSpan also can sum.

Answer 1

Here is sample IsSummable method

public bool IsSummable(Type type)
try
{
  ParameterExpression paramA = Expression.Parameter(type, "a"), paramB = Expression.Parameter(type, "b");
  BinaryExpression addExpression = Expression.Add(paramA, paramB);
  var add = Expression.Lambda(addExpression, paramA, paramB).Compile();
  var v = Activator.CreateInstance(type);
  add.DynamicInvoke(v, v);
  return true;
}
catch
{
  return false;
}
}

and IsAveragable function (what a weird name:) )

public bool IsAveragable(Type type)
try
{
  ParameterExpression paramA = Expression.Parameter(type, "a"), paramB = Expression.Parameter(type, "b");
        // add the parameters together
        BinaryExpression addExpression = Expression.Add(paramA, paramB);
        var avg = Expression.Parameter(typeof(int), "b");
        var conv  = Expression.Convert(avg,type);
        BinaryExpression divideExpression = Expression.Divide(paramA, conv);
        // compile it
        var add = Expression.Lambda(addExpression, paramA, paramB).Compile();
        var divide = Expression.Lambda(divideExpression, paramA, avg).Compile();
        var v = Activator.CreateInstance(type);
        add.DynamicInvoke(v, v);
        divide.DynamicInvoke(v, 1);
                    return true;
}
catch
{
  return false;
}
}

part of the code taken from MiscUtil by Marc Gravell