`var` vs. `dynamic` in C#: Comparing Implicitly Typed Variables

Understand the key differences between C#'s `var` (compile-time type checking) and `dynamic` (runtime type checking) keywords for declaring implicitly typed variables. This guide clarifies their behavior, advantages (conciseness, flexibility), disadvantages (potential runtime errors), and best practices for using them effectively.

`var` vs. `dynamic` in C#: Understanding the Differences

Both `var` and `dynamic` are used in C# to declare variables without explicitly specifying their type. However, they differ significantly in how and when type checking occurs.

`var` (Implicitly Typed Local Variables)

The `var` keyword (introduced in C# 3.0) allows you to declare a local variable without specifying its type. The compiler infers the type from the variable's initializer (the value assigned to it). Type checking happens at compile time.

Syntax

var message = "Hello!"; // message is inferred as string
var count = 10;        // count is inferred as int

Important Notes

• The `var` keyword must be initialized when declared.
• The type is determined at compile time, so you get compile-time type checking and IntelliSense support in your IDE.
• `var` only works for local variables; it can't be used for fields or method parameters.

`dynamic` (Dynamic Typing)

The `dynamic` keyword (introduced in C# 4.0) allows you to declare variables whose type is checked at runtime instead of compile time. This means type checking is deferred until the program executes. This flexibility comes at the cost of potentially missing compile-time errors. IntelliSense support is limited for dynamic variables.

Syntax

dynamic value = 10; // type is not specified
dynamic message = "Hello"; // type is not specified

Important Notes

• Dynamic variables do not need to be initialized at declaration.
• Type checking is performed at runtime; errors are not caught during compilation.
• IntelliSense support in the IDE is limited because the compiler does not know the type during compilation.

Key Differences: `var` vs. `dynamic`

Example: Type Inference vs. Runtime Type Determination

var a = 10;  //Type checking at compile time
a = "test"; //Compiler error
dynamic b = 10; //Type checking at runtime
b = "test"; //No compiler error, potential runtime error if you use b as an integer