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How to: Create and Use C# DLLs (C# Programming Guide)

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

  • MathLibrary.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 UtilityMethods.

  • 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 UtilityMethods.

  • TestCode.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.

// File: Add.cs  
namespace UtilityMethods
    public class AddClass 
        public static long Add(long i, long j) 
            return (i + j);
// File: Mult.cs 
namespace UtilityMethods 
    public class MultiplyClass
        public static long Multiply(long x, long y) 
            return (x * y); 
// File: TestCode.cs 

using UtilityMethods;

class TestCode
    static void Main(string[] args) 
        System.Console.WriteLine("Calling methods from MathLibrary.DLL:");

        if (args.Length != 2)
            System.Console.WriteLine("Usage: TestCode <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);

        System.Console.WriteLine("{0} + {1} = {2}", num1, num2, sum);
        System.Console.WriteLine("{0} * {1} = {2}", num1, num2, product);
/* Output (assuming 1234 and 5678 are entered as command line args):
    Calling methods from MathLibrary.DLL:
    1234 + 5678 = 6912
    1234 * 5678 = 7006652        

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 enables you to use the unqualified class names to reference the DLL methods at compile time, as follows:

MultiplyClass.Multiply(num1, num2);

Otherwise, you have to use the fully qualified names, as follows:

UtilityMethods.MultiplyClass.Multiply(num1, num2);


To run the program, enter the name of the EXE file, followed by two numbers, as follows:

TestCode 1234 5678

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

csc /target:library /out:MathLibrary.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, TestCode.exe, use the following command line:

csc /out:TestCode.exe /reference:MathLibrary.DLL TestCode.cs

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

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