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Creating and Using C# DLLs

A dynamic linking library (DLL) is linked to your program at run time. To demonstrate building and using a DLL, consider the following scenario:

  • MyLibrary.DLL: The library file that contains the methods to be called at run time. In this example, the DLL contains two methods, Add and Multiply.
  • Add.cs: The source file that contains the method Add(long i, long j). It returns the sum of its parameters. The class AddClass that contains the method Add is a member of the namespace MyMethods.
  • Mult.cs: The source code that contains the method Multiply(long x, long y). It returns the product of its parameters. The class MultiplyClass that contains the method Multiply is also a member of the namespace MyMethods.
  • MyClient.cs: The file that contains the Main method. It uses the methods in the DLL file to calculate the sum and the product of the run-time arguments.

Source files

File: Add.cs

// Add two numbers
using System; 
namespace MyMethods 
   public class AddClass 
      public static long Add(long i, long j) 

File: Mult.cs

// Multiply two numbers
using System; 
namespace MyMethods 
   public class MultiplyClass 
      public static long Multiply(long x, long y) 
         return (x*y); 

File: MyClient.cs

// Calling methods from a DLL file
using System; 
using MyMethods;
class MyClient 
   public static void Main(string[] args) 
      Console.WriteLine("Calling methods from MyLibrary.DLL:"); 
      if (args.Length != 2) 
         Console.WriteLine("Usage: MyClient <num1> <num2>"); 
      long num1 = long.Parse(args[0]); 
      long num2 = long.Parse(args[1]); 
      long sum = AddClass.Add(num1, num2);
      long product = MultiplyClass.Multiply(num1, num2);
      Console.WriteLine("The sum of {0} and {1} is {2}",
                   num1, num2, sum); 
      Console.WriteLine("The product of {0} and {1} is {2}", 
                  num1, num2, product); 

This file contains the algorithm that uses the DLL methods, Add and Multiply. It starts with parsing the arguments entered from the command line, num1 and num2. Then it calculates the sum by using the Add method on the AddClass class, and the product by using the Multiply method on the MultiplyClass class.

Notice that the using directive at the beginning of the file allows you to use the unqualified class names to reference the DLL methods at compile time, for example:

MultiplyClass.Multiply(num1, num2); 

Otherwise, you have to use the fully qualified names, for example:

MyMethods.MultiplyClass.Multiply(num1, num2); 


To build the file MyLibrary.DLL, compile the two files Add.cs and Mult.cs using the following command line:

csc /target:library /out:MyLibrary.DLL Add.cs Mult.cs

The /target:library compiler option tells the compiler to output a DLL instead of an EXE file. The /out compiler option followed by a file name is used to specify the DLL file name. Otherwise, the compiler uses the first file (Add.cs) as the name of the DLL.

To build the executable file, MyClient.exe, use the following command line:

csc /out:MyClient.exe /reference:MyLibrary.DLL MyClient.cs

The /out compiler option tells the compiler to output an EXE file and specifies the name of the output file (MyClient.exe). This compiler option is optional. The /reference compiler option specifies the DLL file(s) that this program uses.


To run the program, enter the name of the EXE file, followed by two numbers, for example:

MyClient 1234 5678


Calling methods from MyLibrary.DLL:
The sum of 1234 and 5678 is 6912
The product of 1234 and 5678 is 7006652

See Also

C# Language Tour | Libraries Tutorial

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