How do I make parameter that will accept (x => x.Value) like .Sum(x => x.Value) or .Average(x => x.Value)

Question

How do I make parameter that will accept (x => x.Value) like .Sum(x => x.Value) or .Average(x => x.Value). I'm thinking I need it to be a Func<> but I'm not sure. I don't know what the method signature should look.

Here's what my current extension method looks like:

public static double StdDev(this IEnumerable<double> values)
{
    //code
}

Here's my overload extension so far based (I know this isn't right):

public static double StdDev(this IEnumerable<double> values, Func<double, bool> filter)
{
    IEnumerable<double> data;
    if (filter != null)
    {
        data = values.Where(filter);
    }
    else
    {
        data = values;
    }
    return data.StdDev();
}

Here's my unit test:

[TestClass]
public class IEnumerableExtensions_StdDev
{
    [TestMethod]
    public void StdDev_Success_Lambda()
    {
        var dataSet = new List<TestType> {new TestType(35.2), new TestType(54.43)};

        //This Line compiles fine but the following one does not.
        var test = dataSet.Sum(x => x.Value);
        var stdev = dataSet.StdDev(x => x.Value);

        stdev.Should().BeInRange(13.5, 13.6);
    }
}

public class TestType
{
    public double Value { get; set; }

    public TestType(double value)
    {
        Value = value;
    }
}

The unit test does not compile. It gives me an error that reads:

'List' does not contain a definition for 'StdDev' and the best extension method overload 'IEnumerableExtensions.StdDev(IEnumerable, Func<double, bool>)' requires a receiver of type 'IEnumerable'

I don't want to change the test I want to change the overload method signature and/or body but I don't understand what I need to change in my code to make this work.

score 0 · Answer 1 · answered Apr 11 '21 at 01:20

You can use a second parameter with a default value that is null. I'm pretty sure the C# compiler translates it into an overloaded method, but regardless you can accomplish it with a single method in your code. You will need to account for that default value in your code.

public static double StdDev(this IEnumerable<double> values, Func<double, bool> filter = null)
{
    List<double> data;
    if(filter != null)
    {
        data = values.Where(x => filter(x)).ToList();
    }
    else
    {
        data = values.ToList();
    }
    
    //code
}

How do I make parameter that will accept (x => x.Value) like .Sum(x => x.Value) or .Average(x => x.Value)

1 Answers1