c# - Local variable with same name as instance variable = unexpected results

Question

ASP.NET 4.0 Webforms project. I have the following in my code-behind.

public partial class _Default : System.Web.UI.Page
{
    private string testVar;

    protected override void OnInit(EventArgs e)
    {
        string testVar = "test";
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        var whatsTheValue = testVar;
    }
}

I'm setting a break point inside each method. When the local variable, testVar, is set in OnInit, if I quick watch the instance variable, it also has the value "test". When I play through to Page_Load, the instance variable's value is null.

I ran across this by accident but the behavior is confusing to me. I'm actually surprised that it compiles. I would have expected to see some sort of warning about having two variables with the same name. That being said, it's even more confusing to me that the instance variable picks up the assignment in OnInit, but then immediately loses it when that method is exited.

Can someone explain this behavior to me?

Answer 1

BrokenGlass's answer is of course correct; you are not looking at the field in the first place. The local hides the field.

However, I note that no one has addressed this portion of your question:

I'm actually surprised that it compiles. I would have expected to see some sort of warning about having two variables with the same name.

The C# compiler does not issue an error or warning in that case because doing so would create a form of "brittle base class failure". Suppose you have this code:

class B { }

class D : B 
{ 
    void M() 
    { 
        int x;
        ...

where B is made by one team in your organization, and D is made by an entirely different team. Then one day the maintainers of B decides to add a feature that needs a protected field:

class B { protected int x; }

When you recompile D, should it give an error? If it does, then someone on a completely different team just broke your code! C# has been carefully designed to minimize (though not eliminate) this kind of breakage.

Note that C# does give an error if you re-use x to mean two different things in the same code block. For example:

class B { protected int x; }
class D : B 
{ 
    void M() 
    { 
        x = 123; // meaning this.x inherited from B
        if (whatever)
        {
            int x = 10;
            ... 

That's illegal because now in the body of M, the simple name x is used to mean both this.x and local variable x. Because this is potentially confusing and a bad programming practice, we make it an error. However, if you really want that then you can eliminate the error by ensuring that the usages are in *entirely separate blocks:

class B { protected int x; }
class D : B 
{ 
    void M() 
    { 
        {
            x = 123; // meaning this.x inherited from B
        }
        if (whatever)
        {
            int x = 10;

Now the blocks do not overlap and therefore the compiler assumes that you know what you are doing and allows the code to compile.