What's the Difference Between `var` and `dynamic` in C#?

In-depth understanding of the differences between dynamic and var in C# 4

Many developers have come across the keywords "dynamic" and "var" when exploring the new features of C# 4. Although these two keywords look similar, their functions are completely different and can significantly affect your coding experience.

Let’s uncover the differences between them:

var: Implicit type helper

var, short for "variable", is a type inference keyword introduced in C# 3.0. It simplifies variable declaration by allowing the compiler to infer the type based on the assigned value. For example:

var s = "abc";

Here, the compiler understands that s should be a string because it is assigned the value "abc". The scope of var is local, which means you can only use it with local variables.

dynamic: Runtime magic

dynamic, introduced in C# 4.0, is a fundamental change. Unlike var, it allows you to bypass type checking during compilation, leaving it to runtime. This dynamic nature brings flexibility and potential risks:

dynamic s = "abc";

In this case, the type of s is unknown at compile time, but is checked at runtime to ensure that it has the necessary properties and methods. dynamic Variables provide flexibility in scenarios such as working with COM objects or external libraries with runtime-defined members.

Code comparison: an illustrative example

To further illustrate these differences, consider the following code snippet:

// 无 `dynamic`
var s = "abc";
Console.WriteLine(s.Length);

// 使用 `dynamic`
dynamic s = "abc";
Console.WriteLine(s.Length);

In both cases, the Length attribute is called. However, with var the type checking happens at compile time, while with dynamic it happens at runtime.

Conclusion

dynamic and var are both valuable tools in a C# developer’s toolbox. var Simplified type inference, making the code more concise and readable. dynamic, on the other hand, unlocks dynamic behavior by deferring type checking to runtime, providing greater flexibility, but requires careful handling to avoid runtime errors.

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