`#if DEBUG` vs. `[Conditional('DEBUG')]`: Which Conditional Compilation Approach Should You Choose?

#if DEBUG vs. [Conditional("DEBUG")]: A smart choice

In large-scale project development, the choice of the two conditional compilation methods #if DEBUG and [Conditional("DEBUG")] is crucial. Understanding the subtle differences between the two is critical to ensuring optimal performance and code clarity.

#if DEBUG: Exclude code

from IL

#if DEBUG Conditionally compile code based on the presence or absence of the DEBUG symbol during compilation. The code within the #if DEBUG block exists in the source code, but is excluded from the intermediate language (IL) when compiled for release mode (i.e., without DEBUG symbols). This results in significant performance improvements since unnecessary instructions are not included in the IL.

[Conditional("DEBUG")]: Omit calling

at compile time

In contrast, [Conditional("DEBUG")] modifies methods, specifying that they should be included in the IL regardless of the presence of the DEBUG symbol. However, calls to methods marked with [Conditional("DEBUG")] are omitted at compile time unless the DEBUG symbol is present in the calling assembly. This allows the method to exist in compiled code without affecting performance in release builds.

Choose the right method

The choice of two methods depends on the intended use:

• #if DEBUG: is suitable for code that should be completely excluded from release builds (e.g. debug logs, performance counters).
• [Conditional("DEBUG")]: is for methods that should be present in all versions, but only called when necessary (e.g. validation checks, parameter validation).

Example use case

#if DEBUG:

<code class="language-csharp">#if DEBUG
    public void SetPrivateValue(int value) { ... }
#endif</code>

In this case, the code setting the private value will only be compiled if the DEBUG symbol is present, keeping release builds clean.

[Conditional("DEBUG")]:

<code class="language-csharp">[Conditional("DEBUG")]
protected void VerifyPropertyName(String propertyName) { ... }</code>

This method is always present in the compiled IL, but a call to it is only included if the DEBUG symbol is present in the calling assembly.

Avoid using [Conditional("DEBUG")] for conditional nesting

With [Conditional("DEBUG")], the call to the method is omitted at compile time, even if the call itself is inside a #if DEBUG block. This avoids the need for cumbersome nesting of conditions:

<code class="language-csharp">#if DEBUG
    public void DoSomething() { }
#endif

public void Foo()
{
#if DEBUG
        DoSomething(); // 这种方法很繁琐，而且视觉上不美观
#endif
}</code>

Comparison:

<code class="language-csharp">[Conditional("DEBUG")]
public void DoSomething() { }

public void Foo()
{
    DoSomething(); // 代码更简洁，并且只有在定义了 DEBUG 时才包含调用
}</code>

Conclusion

Understanding the subtle differences between #if DEBUG and [Conditional("DEBUG")] enables developers to make informed choices for conditional compilation. By choosing the right approach, projects can benefit from higher performance, cleaner code, and shorter development times.

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