Why Prefer 'variable != null' over 'null != variable' in C#?

Why use "null != variable" instead of "variable != null" in C#?

In C#, you often see the use of "null != variable" instead of "variable != null" in conditional statements. This may raise questions about the difference in execution speed or the advantage of the former.

Execution speed difference

In C#, the order of operands in a conditional statement does not affect execution speed. "null != variable" and "variable != null" evaluate to the same result and have the same computational cost.

Legacy issues of C language

The reason for using "null != variable" stems from historical habits in the C programming language. In C, the assignment operator ("=") and the equality comparison operator ("==") have different precedence, which can lead to unexpected behavior if the operands are not in the correct order.

C language example:

<code class="language-c">if (x = 5) // 赋值，结果为真</code>

To avoid this situation, C programmers often use the reverse form "if (5 == x)" to ensure correct behavior.

This problem does not exist in C#

In C#, "==" has higher precedence than "=". This removes potential ambiguity and eliminates the need to use "null != variable" for correctness.

Advantages of "variable != null"

Using "variable != null" has several advantages over "null != variable":

• Better readability: It's more natural and easier to understand, especially for people unfamiliar with the quirks of the C language.
• Eliminate potential confusion: Use "variable != null" to prevent any confusion or accidental assignment.
• Conforms to C# best practices: It complies with C# coding conventions and recommended practices.

Therefore, for clarity and in line with best practices, it is recommended to use "variable != null" instead of "null != variable" in C#.

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