What is the difference between declaration and definition in C#?

In C#, declaration means specifying the type and name of a variable, method, or class without providing an initial value or implementation. Definition (or initialization) means providing an actual value or implementation to what was declared.

This distinction applies to variables, arrays, methods, and classes. Understanding the difference helps write cleaner code and avoid compilation errors.

Syntax

Following is the syntax for declaration −

datatype variableName;

Following is the syntax for definition/initialization −

variableName = value;
// or combined declaration and initialization
datatype variableName = value;

Variable Declaration vs Definition

Example

using System;

class Program {
   static void Main() {
      // Declaration only
      int x;
      string name;
      
      // Definition/Initialization
      x = 10;
      name = "John";
      
      // Combined declaration and definition
      int y = 20;
      string city = "New York";
      
      Console.WriteLine("x = " + x);
      Console.WriteLine("name = " + name);
      Console.WriteLine("y = " + y);
      Console.WriteLine("city = " + city);
   }
}

The output of the above code is −

x = 10
name = John
y = 20
city = New York

Array Declaration vs Definition

Example

using System;

class Program {
   static void Main() {
      // Declaration only
      int[] numbers;
      
      // Definition/Initialization
      numbers = new int[3];
      numbers[0] = 100;
      numbers[1] = 200;
      numbers[2] = 300;
      
      // Combined declaration and definition
      string[] names = {"Alice", "Bob", "Charlie"};
      
      Console.WriteLine("Array elements:");
      for(int i = 0; i < numbers.Length; i++) {
         Console.WriteLine("numbers[" + i + "] = " + numbers[i]);
      }
      
      Console.WriteLine("Names array:");
      for(int i = 0; i < names.Length; i++) {
         Console.WriteLine("names[" + i + "] = " + names[i]);
      }
   }
}

The output of the above code is −

Array elements:
numbers[0] = 100
numbers[1] = 200
numbers[2] = 300
Names array:
names[0] = Alice
names[1] = Bob
names[2] = Charlie

Method Declaration vs Definition

In C#, methods are typically declared and defined together, but in interfaces and abstract classes, you can have method declarations without definitions −

Example

using System;

interface ICalculator {
   // Method declaration only (no implementation)
   int Add(int a, int b);
}

class Calculator : ICalculator {
   // Method definition (implementation provided)
   public int Add(int a, int b) {
      return a + b;
   }
}

class Program {
   static void Main() {
      Calculator calc = new Calculator();
      int result = calc.Add(5, 3);
      Console.WriteLine("Result: " + result);
   }
}

The output of the above code is −

Result: 8

Key Differences

Conclusion

Declaration establishes the type and name of variables, methods, or classes, while definition provides the actual value or implementation. In C#, you can declare and define separately or combine both operations in a single statement for cleaner code.