Why did I get the compile error "Use of unassigned local variable"?

Local variables aren't initialized. You have to manually initialize them.

Members are initialized, for example:

public class X
{
    private int _tmpCnt; // This WILL initialize to zero
    ...
}

But local variables are not:

public static void SomeMethod()
{
    int tmpCnt;  // This is not initialized and must be assigned before used.

    ...
}

So your code must be:

int tmpCnt = 0;  
if (name == "Dude")  
   tmpCnt++;  

So the long and the short of it is, members are initialized, locals are not. That is why you get the compiler error.

The following categories of variables are classified as initially unassigned:

• Instance variables of initially unassigned struct variables.
• Output parameters, including the this variable of struct instance constructors.
• Local variables , except those declared in a catch clause or a foreach statement.

The following categories of variables are classified as initially assigned:

• Static variables.
• Instance variables of class instances.
• Instance variables of initially assigned struct variables.
• Array elements.
• Value parameters.
• Reference parameters.
• Variables declared in a catch clause or a foreach statement.

Default assignments apply to class members, but not to local variables. As Eric Lippert explained it in this answer, Microsoft could have initialized locals by default, but they choose not to do it because using an unassigned local is almost certainly a bug.

Local variables don't have a default value.

They have to be definitely assigned before you use them. It reduces the chance of using a variable you think you've given a sensible value to, when actually it's got some default value.