It really depends on what you're going for:
#if DEBUG: The code in here won't even reach the IL on release.[Conditional("DEBUG")]: This code will reach the IL, however calls to the method will be omitted unless DEBUG is set when the caller is compiled.Personally I use both depending on the situation:
Conditional("DEBUG") Example: I use this so that I don't have to go back and edit my code later during release, but during debugging I want to be sure I didn't make any typos. This function checks that I type a property name correctly when trying to use it in my INotifyPropertyChanged stuff.
[Conditional("DEBUG")]
[DebuggerStepThrough]
protected void VerifyPropertyName(String propertyName)
{
if (TypeDescriptor.GetProperties(this)[propertyName] == null)
Debug.Fail(String.Format("Invalid property name. Type: {0}, Name: {1}",
GetType(), propertyName));
}
You really don't want to create a function using #if DEBUG unless you are willing to wrap every call to that function with the same #if DEBUG:
#if DEBUG
public void DoSomething() { }
#endif
public void Foo()
{
#if DEBUG
DoSomething(); //This works, but looks FUGLY
#endif
}
versus:
[Conditional("DEBUG")]
public void DoSomething() { }
public void Foo()
{
DoSomething(); //Code compiles and is cleaner, DoSomething always
//exists, however this is only called during DEBUG.
}
#if DEBUG example: I use this when trying to setup different bindings for WCF communication.
#if DEBUG
public const String ENDPOINT = "Localhost";
#else
public const String ENDPOINT = "BasicHttpBinding";
#endif
In the first example, the code all exists, but is just ignored unless DEBUG is on. In the second example, the const ENDPOINT is set to "Localhost" or "BasicHttpBinding" depending on if DEBUG is set or not.
Update: I am updating this answer to clarify an important and tricky point. If you choose to use the ConditionalAttribute, keep in mind that calls are omitted during compilation, and not runtime. That is:
MyLibrary.dll
[Conditional("DEBUG")]
public void A()
{
Console.WriteLine("A");
B();
}
[Conditional("DEBUG")]
public void B()
{
Console.WriteLine("B");
}
When the library is compiled against release mode (i.e. no DEBUG symbol), it will forever have the call to B() from within A() omitted, even if a call to A() is included because DEBUG is defined in the calling assembly.