C# var vs dynamic: Differences, Usage, and Best Practices
var is a statically typed variable where the type is determined at compile time, while dynamic is a dynamically typed variable where type checking happens at runtime.
In C#, both var and dynamic allow flexibility in variable declaration, but they behave very differently. The var keyword enables type inference, meaning the compiler determines the type based on the assigned value. Once assigned, the type cannot change. On the other hand, dynamic bypasses compile-time type checking, allowing operations to be resolved at runtime. This makes dynamic more flexible but also more error-prone compared to var.
When to Use var vs dynamic?
Use var when:
• The type is obvious from the assignment
• You want cleaner and shorter code
• You need compile-time type safety
• Working with LINQ queries or complex types
Use dynamic when:
• Working with external APIs (e.g., JSON, COM objects)
• Type is not known at compile time
• You need runtime flexibility
• Interoperating with dynamic languages
How to Use var and dynamic?
var Example (Compile-Time Type)
using System;
class Program
{
static void Main()
{
var number = 10; // int inferred
var name = "Alice"; // string inferred
Console.WriteLine(number);
Console.WriteLine(name);
}
}
dynamic Example (Runtime Type)
using System;
class Program
{
static void Main()
{
dynamic value = 10;
Console.WriteLine(value);
value = "Hello";
Console.WriteLine(value);
}
}
dynamic Runtime Error Example
using System;
class Program
{
static void Main()
{
dynamic value = "Hello";
Console.WriteLine(value.Length); // OK
value = 10;
Console.WriteLine(value.Length); // Runtime error
}
}
var vs dynamic Comparison
| Feature | var | dynamic |
|---|---|---|
| Type Resolution | Compile-time | Runtime |
| Type Safety | Strongly typed | Weakly typed |
| Error Detection | At compile time | At runtime |
| Flexibility | Low | High |
| Performance | Faster | Slower due to runtime binding |
Advantages and Disadvantages
| Keyword | Advantages | Disadvantages |
|---|---|---|
| var | Clean code, compile-time safety, better performance | Type may be less explicit to beginners |
| dynamic | High flexibility, useful for dynamic data | No compile-time checking, runtime errors possible |
Similar and Alternative Options
| Option | Description | When to Use |
|---|---|---|
| Explicit types | Direct type declaration (int, string) | Best for clarity and safety |
| object | Base type for all types | When type is unknown but static casting is needed |
| Reflection | Runtime type inspection | Advanced dynamic scenarios |
| JSON parsing | Dynamic data handling | API responses |
Common Mistakes
• Using dynamic without understanding runtime risks
• Assuming var means dynamic typing (it does not)
• Overusing dynamic instead of proper types
• Ignoring compile-time safety benefits of var
• Using dynamic in performance-critical code
• Not handling runtime exceptions with dynamic objects
