In C#, both the static and readonly keywords are used to define fields, but they serve different purposes and are used in distinct scenarios. Understanding when and how to use these keywords is essential for writing efficient and maintainable code. Let’s dive into the differences and specific use cases for each.

Please read this blog: Sealed Class in C#

Static Keyword

The static keyword in C# is used to declare a member of a class that belongs to the type itself rather than to any specific object. This means that static members are shared among all instances of the class. Here are the main points and use cases for the static keyword:

• Class-Level Scope: static members are accessed using the class name rather than an instance of the class.
• Shared Data: Since static members are shared across all instances, they are useful for data or methods that should be common to all objects.
• Memory Efficiency: Only one copy of a static member exists, regardless of how many instances of the class are created.

When to use static Keyword

We can use static keywords for below uses:

• Methods that do not depend on instance data and are stateless can be made static.
• Configuration settings or constant values that are common across the application can be defined as static
• The Singleton design pattern often uses a static instance to ensure only one instance of the class exists.

Readonly Keyword

The readonly keyword is used to declare fields that can only be assigned a value once, either at the time of declaration or in the constructor of the class. Unlike static, readonly fields are instance-specific and can vary between instances.

• Immutable After Initialization: Once a readonly field is initialized, its value cannot be changed.
• Instance-Specific: Each instance of the class can have different values for readonly fields.

When to use Readonly Keyword

We should use readonly on below cases:

• When you want to set a configuration or constant value at runtime ensure it does not change thereafter.
• When injecting dependencies, readonly fields ensure that the dependencies are immutable once set by the constructor.
• When we have data that is constant after initialization but can differ between instances, such as the unique identifier of an object.

Key Differences between Static and Readonly Keyword

Below are some of the key differences between static and readonly keywords.

Scope:

• static: Class-level scope. Shared across all instances.
• readonly: Instance-level scope. Each instance can have a different value.

Initialization:

• static: Can be initialized at declaration or in a static constructor.
• readonly: Can be initialized at declaration or in an instance constructor.

Mutability:

• static: Can be mutable or immutable, depending on the design.
• readonly: Immutable after initialization.

Conclusion

Choosing between static and readonly depends on the specific requirements of the application. We use static keyword when we need a member to be shared across all instances of a class. On the other hand, we use readonly when we need instance-specific, immutable fields that are set only once.

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