C# `var` vs. `dynamic`: What's the Real Difference?

A detailed comparison of "dynamic" and "var" in C#

Efficient C# development requires understanding the subtle differences between "dynamic" and "var". Both keywords introduce type inference, but their impact goes far beyond simple shorthand notation.

Static type and dynamic type

• var: A statically typed variable whose type is inferred by the compiler based on its initialization. It's actually shorthand for explicitly declaring a variable's type, with the compiler verifying type compatibility during compilation. For example:

<code class="language-C#">var s = "abc";
Console.WriteLine(s.Length);</code>

is equivalent to:

<code class="language-C#">string s = "abc";
Console.WriteLine(s.Length);</code>

• dynamic: A dynamically typed variable whose type is unknown at compile time and is only determined when the program is running. It allows access to properties and methods that may not exist at compile time because types are resolved at runtime. For example:

<code class="language-C#">dynamic s = "abc";
Console.WriteLine(s.Length);</code>

This code compiles because the compiler doesn't know the actual type of the variable, but will fail at runtime if the object assigned to s doesn't have a Length property.

Runtime Behavior Analysis

• var: Since the types are known at compile time, static checks are performed to ensure type compatibility and any errors are detected early.
• dynamic: Type checking is deferred to runtime, allowing greater flexibility, but may also throw errors during execution.

Summary

"var" simplifies code and improves readability by eliminating duplicate type declarations, while "dynamic" enables developers to work with dynamic or late-bound objects and interact with external systems that may have unknown types. Understanding the subtle differences between these two keywords enables effective coding practices and helps avoid potential pitfalls.

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