Var vs. Dynamic in C# 4: What's the Difference?

Detailed explanation of the difference between var and dynamic in C# 4

C# 4 introduced the "var" and "dynamic" keywords, both of which aim to simplify code writing, but they have different features and application scenarios, often confusing developers.

Var keyword

"var" is a type inference feature that allows you to implicitly specify the type of a variable based on its assigned value. This eliminates the need to explicitly declare the type name, making code cleaner. For example:

<code class="language-csharp">var s = "Hello";</code>

This is equivalent to:

<code class="language-csharp">string s = "Hello";</code>

Dynamic keyword

Unlike "var", "dynamic" allows you to deal with objects whose properties and methods are not known at compile time. It supports dynamic dispatch, and the compiler only checks the existence of the member at runtime. This approach is particularly useful when interacting with dynamic systems (such as JavaScript or COM), since the properties of an object may change dynamically. For example:

<code class="language-csharp">dynamic s = "Hello";
Console.WriteLine(s.Length); // 调用 string.Length 属性
Console.WriteLine(s.FlibbleBananaSnowball); // 抛出运行时错误</code>

Unlike "var" which remains strongly typed at runtime, "dynamic" allows late binding and member checking. This flexibility can be advantageous, but it can also introduce runtime errors when the object does not have the expected members.

Summary

"var" simplifies code through type inference, while "dynamic" provides greater flexibility in handling dynamic objects at runtime. Understanding the difference between these two keywords is critical to taking full advantage of C# 4’s streamlined coding experience.

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