C# compiling optimizations: null coalescing operator - UPDATED - Reflector's bug?

Question

Greetings! I am slightly confused about how does C# compiler perform its optimizations.
I have written the following getter to make up "lazy" initialization, and default value in case of null:

Static class Helper:

private static string host;  
public static string Host  
{        
    get  
    {  
        return host ?? (host= (ConfigurationManager.AppSettings["Host"] ?? "host.ru"));  
    }  
}

Here is the result of disassembling by Reflector:

public static string Host 
{  
    get  
    {  
        if (Helper.host == null)  
        {  
            string host = Helper.host;  
        }  
        return (Helper.host = ConfigurationManager.AppSettings["Host"] ?? "host.ru");  
    }  
}

Looks like it would work in other way than assumed...

UPDATE

    private static string host;
    public static string Host
    {
        get
        {
            return host ?? (host = (GetVal() ?? "default"));
        }
    }
    static void Main(string[] args)
    {

        Console.WriteLine(Host);
        host = "overwritten";
        Console.WriteLine(Host);
    }
    static string GetVal()
    {
        return "From config";
    }

Works correctly (From config, overwritten), but Reflector shows the same:

public static string Host
{
    get
    {
        if (Program.host == null)
        {
            string host = Program.host;
        }
        return (Program.host = GetVal() ?? "default");
    }
}

Answer 1

This looks like a bug in Reflector's C# disassembly.

Starting with this code:

public static string _test;
public static string _setting;

public static string Test_1
{
    get { return _test ?? (_setting ?? "default"); }
}

Reflector shows this C# disassembly:

public static string Test_1
{
    get
    {
        return (_test ?? (_setting ?? "default"));
    }
}

and the corresponding IL:

.method public hidebysig specialname static string get_Test_1() cil managed
{
    .maxstack 8
    L_0000: ldsfld string ConsoleApplication1.Program::_test
    L_0005: dup 
    L_0006: brtrue.s L_0017
    L_0008: pop 
    L_0009: ldsfld string ConsoleApplication1.Program::_setting
    L_000e: dup 
    L_000f: brtrue.s L_0017
    L_0011: pop 
    L_0012: ldstr "default"
    L_0017: ret 
}

I am not an IL expert, but this is my take on it:

• L_0000:ldsfld pushes _test onto the evaluation stack
• L_0005:dup copies the value (_test) that is topmost on the evaluation stack and pushes that onto the stack.
• L_0006:brtrue.s pops the value created by dup off the stack and jumps to L_0017 if it is not null.
• L_0008:pop at this point, _test is null, so pop that value off the stack.

and it continues to evaluate _setting in a similar fashion, finally returning "default" if _setting is also null.

Now, if we add an assignment into the code like this:

public static string Test_2
{
    get { return _test ?? (_test = (_setting ?? "default")); }
}

Reflector shows this C# disassembly:

public static string Test_2
{
    get
    {
        if (_test == null)
        {
            string text1 = _test;
        }
        return (_test = _setting ?? "default");
    }
}

which is not correct (if _test is not null, instead of returning _test, it assigns _setting or "default" to _test and then returns).

However, the IL dissassembly looks like the IL for Test_1, with a couple of extra instructions at L_0017 and L_0018 to do the assignment.

.method public hidebysig specialname static string get_Test_2() cil managed
{
    .maxstack 8
    L_0000: ldsfld string ConsoleApplication1.Program::_test
    L_0005: dup 
    L_0006: brtrue.s L_001d
    L_0008: pop 
    L_0009: ldsfld string ConsoleApplication1.Program::_setting
    L_000e: dup 
    L_000f: brtrue.s L_0017
    L_0011: pop 
    L_0012: ldstr "default"
    L_0017: dup 
    L_0018: stsfld string ConsoleApplication1.Program::_test
    L_001d: ret 
}

Finally, if you copy Reflector's C# dissembly and run it against the original, you'll see it produces different results.

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            _test = "Test";
            Console.WriteLine(Test_2);
            Console.WriteLine(Reflector_Test_2);
            Console.ReadLine();
        }

        public static string _test;
        public static string _setting;

        public static string Test_1
        {
            get { return _test ?? (_setting ?? "default"); }
        }

        public static string Test_2
        {
            get { return _test ?? (_test = (_setting ?? "default")); }
        }

        public static string Reflector_Test_2
        {
            get
            {
                if (_test == null)
                {
                    string text1 = _test;
                }
                return (_test = _setting ?? "default");
            }
        }
    }
}

Outputs

Test
default