September 21, 2014
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.NET Tip: Using a Parameter Array in Your Functions

  • June 14, 2006
  • By Eric Smith
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Most functions require a set list of parameters to operate. However, you'll create some functions that need a variable list of parameters. A good example of this is the AppendFormat method of the StringBuilder class. Depending on the format string you specify, you may need zero additional arguments, or nine, or any number.

This tip shows you how to create a function that concatenates any number of arguments and returns a single string. The function definition is as follows:

private string Concatenate(string separator, params object[] parts)
{
   System.Text.StringBuilder buffer = new System.Text.StringBuilder();
   string sepValue = "";
   foreach (object o in parts)
   {
      buffer.AppendFormat("{0}{1}", sepValue, o);
      sepValue = separator;
   }
   return buffer.ToString();
}

You call the function as follows:

string result = Concatenate(" ", "test1", "test2", "test3");

The important thing to notice here is the params keyword in the parameter list. This indicates that the function accepts a variable list of arguments—in this case, an array of objects called parts. The parameter array needs to be the final parameter in the list of parameters, and as shown here, you can have non-parameter array parameters, as well.

If you called this function with just the first string, the first string value would be put in the separator parameter and the parts array would be empty. Although this may not make sense, as long as you have the first string, the call is syntactically valid.

Within the function, you create a StringBuilder object to create the concatenated string. You prepend each parameter array member with the separator string. The first time around, however, you don't have a separator string other than an empty string. This saves you from any messy calculations to determine whether you need to remove the last separator at the end of the string, for instance.

I have an instance of this function that generates a SqlCommand object for me. In that case, I use a trio of parameters in the parameter array: the parameter name, the data type (SqlDbType), and the value. The function verifies that the number of parameters is divisible by three. If it isn't, an exception is thrown.

About the Author

Eric Smith is the owner of Northstar Computer Systems, a Web-hosting company based in Indianapolis, Indiana. He is also a MCT and MCSD who has been developing with .NET since 2001. In addition, he has written or contributed to 12 books covering .NET, ASP, and Visual Basic.






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