问题
I'm getting an unexpected NullReferenceException when I run this code, omitting the fileSystemHelper parameter (and therefore defaulting it to null):
public class GitLog
{
FileSystemHelper fileSystem;
/// <summary>
/// Initializes a new instance of the <see cref="GitLog" /> class.
/// </summary>
/// <param name="pathToWorkingCopy">The path to a Git working copy.</param>
/// <param name="fileSystemHelper">A helper class that provides file system services (optional).</param>
/// <exception cref="ArgumentException">Thrown if the path is invalid.</exception>
/// <exception cref="InvalidOperationException">Thrown if there is no Git repository at the specified path.</exception>
public GitLog(string pathToWorkingCopy, FileSystemHelper fileSystemHelper = null)
{
this.fileSystem = fileSystemHelper ?? new FileSystemHelper();
string fullPath = fileSystem.GetFullPath(pathToWorkingCopy); // ArgumentException if path invalid.
if (!fileSystem.DirectoryExists(fullPath))
throw new ArgumentException("The specified working copy directory does not exist.");
GitWorkingCopyPath = pathToWorkingCopy;
string git = fileSystem.PathCombine(fullPath, ".git");
if (!fileSystem.DirectoryExists(git))
{
throw new InvalidOperationException(
"There does not appear to be a Git repository at the specified location.");
}
}
When I single step the code in the debugger, after I step over the first line (with the ?? operator) then fileSystem still has the value null, as shown in this screen snip (stepping over the next line throws NullReferenceException):
This is not what I expected! I'm expecting the null coalescing operator to spot that the parameter is null and create a new FileSystemHelper(). I have stared at this code for ages and can't see what is wrong with it.
ReSharper pointed out that the field is only used in this one method, so could potentially be converted to a local variable... so I tried that and guess what? It worked. So, I have my fix, but I cannot for the life of me see why the code above doesn't work. I feel like I am on the edge of learning something interesting about C#, either that or I've done something really dumb. Can anyone see what's happening here?
回答1:
I have reproduced it in VS2012 with the following code:
public void Test()
{
TestFoo();
}
private Foo _foo;
private void TestFoo(Foo foo = null)
{
_foo = foo ?? new Foo();
}
public class Foo
{
}
If you set a breakpoint at the end of the TestFoo method, you would expect to see the _foo variable set, but it will still show as null in the debugger.
But if you then do anything with _foo, it then appears correctly. Even a simple assignment such as
_foo = foo ?? new Foo();
var f = _foo;
If you step through it, you'll see that _foo shows null until it is assigned to f.
This reminds me of deferred execution behavior, such as with LINQ, but I can't find anything that would confirm that.
It's entirely possible that this is just a quirk of the debugger. Perhaps someone with MSIL skills can shed some light on what is happening under the hood.
Also interesting is that if you replace the null coalescing operator with it's equivalent:
_foo = foo != null ? foo : new Foo();
Then it does not exhibit this behavior.
I am not an assembly/MSIL guy, but just taking a look at the dissasembly output between the two versions is interesting:
_foo = foo ?? new Foo();
0000002d mov rax,qword ptr [rsp+68h]
00000032 mov qword ptr [rsp+28h],rax
00000037 mov rax,qword ptr [rsp+60h]
0000003c mov qword ptr [rsp+30h],rax
00000041 cmp qword ptr [rsp+68h],0
00000047 jne 0000000000000078
00000049 lea rcx,[FFFE23B8h]
00000050 call 000000005F2E8220
var f = _foo;
00000055 mov qword ptr [rsp+38h],rax
0000005a mov rax,qword ptr [rsp+38h]
0000005f mov qword ptr [rsp+40h],rax
00000064 mov rcx,qword ptr [rsp+40h]
00000069 call FFFFFFFFFFFCA000
0000006e mov r11,qword ptr [rsp+40h]
00000073 mov qword ptr [rsp+28h],r11
00000078 mov rcx,qword ptr [rsp+30h]
0000007d add rcx,8
00000081 mov rdx,qword ptr [rsp+28h]
00000086 call 000000005F2E72A0
0000008b mov rax,qword ptr [rsp+60h]
00000090 mov rax,qword ptr [rax+8]
00000094 mov qword ptr [rsp+20h],rax
Compare that to the inlined-if version:
_foo = foo != null ? foo : new Foo();
0000002d mov rax,qword ptr [rsp+50h]
00000032 mov qword ptr [rsp+28h],rax
00000037 cmp qword ptr [rsp+58h],0
0000003d jne 0000000000000066
0000003f lea rcx,[FFFE23B8h]
00000046 call 000000005F2E8220
0000004b mov qword ptr [rsp+30h],rax
00000050 mov rax,qword ptr [rsp+30h]
00000055 mov qword ptr [rsp+38h],rax
0000005a mov rcx,qword ptr [rsp+38h]
0000005f call FFFFFFFFFFFCA000
00000064 jmp 0000000000000070
00000066 mov rax,qword ptr [rsp+58h]
0000006b mov qword ptr [rsp+38h],rax
00000070 nop
00000071 mov rcx,qword ptr [rsp+28h]
00000076 add rcx,8
0000007a mov rdx,qword ptr [rsp+38h]
0000007f call 000000005F2E72A0
var f = _foo;
00000084 mov rax,qword ptr [rsp+50h]
00000089 mov rax,qword ptr [rax+8]
0000008d mov qword ptr [rsp+20h],rax
Based on this, I do think there is some kind of deferred execution happening. The assignment statement in the second example is very small in comparison to the first example.
回答2:
Someone else experienced the same problem in this question. Interestingly it is also using a this._field = expression ?? new ClassName(); format. It could be some kind of issue with the debugger, as writing the value out seemed to produce the correct results for them.
Try adding debug/log code to show the value of the field after assignment to eliminate weirdness in the attached debugger.
来源：https://stackoverflow.com/questions/19352130/why-doesnt-the-null-coalescing-operator-work-in-this-situation