Dynamic vs var in C#: Key Differences and Usage

var is used for implicit typing where the type is determined at compile time, while dynamic defers type checking until runtime, allowing more flexible but less safe code. Use var when the type is known and fixed, and dynamic when you need to work with objects whose type is not known until the program runs.

Quick Comparison

This table summarizes the main differences between var and dynamic in C#.

Aspect	var	dynamic
Type Checking	Compile-time	Runtime
Type Inference	Compiler infers type from assigned value	No inference; treated as dynamic type
Safety	Type-safe, errors caught at compile time	Not type-safe, errors caught at runtime
Use Case	When type is known and fixed	When type is unknown or changes at runtime
Performance	Faster due to compile-time binding	Slower due to runtime binding
IntelliSense Support	Full support in IDE	Limited support in IDE

Key Differences

var is a keyword that tells the compiler to infer the variable's type from the value assigned at compile time. Once inferred, the variable has a fixed type and cannot change. This means the compiler checks all operations on the variable for correctness before the program runs, making var type-safe and efficient.

On the other hand, dynamic is a type that bypasses compile-time type checking. Variables declared as dynamic are resolved at runtime, allowing you to call methods or access properties that the compiler does not verify. This flexibility is useful when working with COM objects, dynamic languages, or JSON, but it sacrifices safety and performance.

In summary, var is for implicit but fixed typing known at compile time, while dynamic is for dynamic typing resolved at runtime.

Code Comparison

Here is an example using var to declare a variable with an inferred type.

csharp

var number = 10;
Console.WriteLine(number + 5);

Output

15

Dynamic Equivalent

The same example using dynamic allows the variable to hold any type and resolves operations at runtime.

csharp

dynamic number = 10;
Console.WriteLine(number + 5);

Output

15

When to Use Which

Choose var when you want the compiler to infer the type and ensure type safety with better performance. It is ideal for most cases where the type is known or obvious.

Choose dynamic when you need flexibility to work with objects whose type is not known until runtime, such as interacting with COM APIs, dynamic JSON data, or scripting languages. Use it sparingly because it can lead to runtime errors and slower code.

Key Takeaways

var infers type at compile time and is type-safe.

dynamic defers type checking to runtime and is more flexible but less safe.

Use var for known types and better performance.

Use dynamic for unknown or changing types at runtime.

Avoid dynamic when type safety and performance are priorities.