Lambda in C# extension method

Question

Suppose I want to write an extension method to dump some data from a T[,] to a CSV:

public static void WriteCSVData<T>(this T[,] data, StreamWriter sw)
{
    for (int row = 0; row < data.GetLength(0); row++)
        for (int col = 0; col < data.GetLength(1); col++)
        {
            string s = data[row, col].ToString();

            if (s.Contains(","))
                sw.Write("\"" + s + "\"");
            else
                sw.Write(s);

            if (col < data.GetLength(1) - 1)
                sw.Write(",");
            else
                sw.WriteLine();
        }
}

which I could call with

using (StreamWriter sw = new StreamWriter("data.csv"))
  myData.WriteCSVData(sw);

but suppose myData is a Complex[,] and I want to write the magnitude of the complex number, not the full value. It would be handy if I could write:

using (StreamWriter sw = new StreamWriter("data.csv"))
  myData.WriteCSVData(sw, d => d.Magnitude);

but I'm not sure how to implement that in the extension method or if it's even possible.

Don't forget to quote any quotes or newlines in your output fields! — Gabe
– Gabe, Commented Sep 15, 2014 at 6:50

Eren Ersönmez · Accepted Answer · 2014-09-15 12:46:32Z

8

You can write an overload of your existing method like this:

public static void WriteCSVData<T, TValue>(this T[,] data, StreamWriter sw, 
    Func<T,TValue> func)
{
    for (int row = 0; row < data.GetLength(0); row++)
        for (int col = 0; col < data.GetLength(1); col++)
        {
            string s = func(data[row, col]).ToString();

            if (s.Contains(","))
                sw.Write("\"" + s + "\"");
            else
                sw.Write(s);

            if (col < data.GetLength(1) - 1)
                sw.Write(",");
            else
                sw.WriteLine();
        }
}

and use it the way you wanted:

using (StreamWriter sw = new StreamWriter("data.csv"))
    myData.WriteCSVData(sw, d => d.Magnitude);

edited Sep 15, 2014 at 12:46

answered Sep 15, 2014 at 6:41

Eren Ersönmez

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2 Comments

Gabe Over a year ago

-1, Your for loop version generates a new line for each row, while your LINQ version generates a new line for each element.

Eren Ersönmez Over a year ago

@Gabe you're right. I must have overlooked that it only writes a new line for the last element of a row. removed.

Sercan · Accepted Answer · 2014-09-15 06:45:54Z

1

Define a delegate with parameter type T and return type string. Add a parameter to method WriteCSVData with type the delegate.

delegate string ExtractValueDelegate<T>(T obj);

public static void WriteCSVData<T>(this T[,] data,ExtractValueDelegte<T> extractor , StreamWriter sw) { ... }

// calling the method
 myData.WriteCSVData(sw, d => d.Magnitude.ToString());

answered Sep 15, 2014 at 6:45

Sercan

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1 Comment

Christoph Fink Over a year ago

There is no need for "manual delegates" anymore. .NET has them built in now - Func<T, string> in this case...

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