Extension method, SumIf on generic List<T>

Question

I need to write a generic extension method for List<T> that conditionally considers each string property of T, then sums a cooresponding decimal property of T if a condition is met. My effort thus far:

// pseudo:  foreach(p in Persons) { if(p.Name == "mort"){sum p.Amount;} }
public static double SumIf<T>(this List<T> ListItems, string targetString, ?strValues?, ?dblValues?)
{
    double sum = 0;
    foreach(T item in ListItems)
    {
        if(item.?strValue? == targetString){ sum += item.?dblValue? ; }
    }
    return sum;
}

Thanks for any guidance

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Mort

Answer 1

You can already .Aggregate a collection using LINQ.  That does the sum=0, and sum += foo parts.  The foo part can be a lambda expression to select the value of foo.

I think your extension method generic function will need to be qualified with a where expression, something that allows you to specify the class that has the .strValue and .dblValue properties.

Show the rest of the syntax that you want to use:

myList.SumIf( "mort", ..., ... );

show what you hope to be able to put where the ...'s are.

Answer 2

A generic "SumIf" would probably take a predicate, and look something like:

public static double SumIf(this IEnumerable<T> collection, Func<T, bool> predicate, Func<T, double> valueFunc)
{
    double total = 0.0;

    foreach(var item in collection)
    {
        if (predicate(item))
            total += valueFunc(item);
    }

    return total;
}

This would then allow you to write something like:

     double total = someCollection.SumIf(item => item.Name == "mort", item => item.Amount);

This works by passing two delegates (instead of one, like many LINQ methods) - the first defines the "if condition" via a predicate, the second tells the method how to extract a double value to sum.

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Reed Copsey, Jr. - http://reedcopsey.com
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