How Does Variable Scope Affect Compilation in C#?

Ambiguity of variable scope in C#

Variable scope in C# can be confusing due to certain language rules. Let’s dig into two code examples to understand the problem:

Code example 1 (compilation error):

<code class="language-c#">public void MyMethod(){
    int i = 10;

    for(int x = 10; x < 20; x++){
        int i = x; // 编译错误：在此作用域内重复声明变量 'i'
        object objX = new object(); // 编译错误：在此作用域内重复声明变量 'objX'
        object objX = new object();
    }
}</code>

Error reason:

• Point 1: Two local variables named 'i' are declared within the same code block (loop body). C# does not allow duplicate declarations with the same name in the same scope.
• Point 2: Another local variable named 'objX' is declared in the same scope as the previous variable of the same name, violating the "identification rules" in C#. This rule states that within a block of code, simple names must always refer to the same entity.

Code example 2 (compiled successfully):

<code class="language-c#">public void MyMethod(){
    for(int x = 10; x < 20; x++){
        int i = x; 
        object objX = new object();
    }
    for(int x = 10; x < 20; x++){
        int i = x;
        object objX = new object();
    }
}</code>

Reason for successful compilation:

In Code Example 2, the loop's "implicit braces" rule creates a separate scope for each loop iteration. This means:

• The two variables named 'i' are declared in different scopes (different iterations of the loop), thus avoiding duplicate declaration errors.
• Two variables named 'objX' are also declared in different scopes, satisfying the "identification rules", allowing compilation to succeed.

These two code examples illustrate the subtle effects of variable scope in C# and the confusion it can cause when not understood correctly.

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