Author avatar

Gaurav Singhal

Know the Scope of Local Variables

Gaurav Singhal

  • Nov 15, 2019
  • 9 Min read
  • 40 Views
  • Nov 15, 2019
  • 9 Min read
  • 40 Views
Languages Frameworks and Tools
C#

Introduction

When a variable is declared in a method of a C# program, its scope is pre-defined and its visibility is defined for the rest of the program. When it's created this way, the variable will be available as long as its method is in execution. However, when the control is passed to another method, its scope ends. This type of variable is called a local variable.

In this guide, we will learn about the scope of local variables and combat the issues related to the position of variable declaration in C# code.

An example of a local variable:

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using System;
public class NewProgram {
   public static void Main(string[] args) {
      int x;
      x = 10; // local variable
      Console.WriteLine("Value:" + x);
   }
}
csharp

Output will be:

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Value:10
console

These variables can be used only within code blocks that are inside that function.

There are other ways to define and use a variable by declaring it in a class (or as a global variable), which allows it to be available to all methods in that class.

Global variables can be accessed from anywhere in a class or namespace. C# does not directly support global variables, but the functionality of global variables can be achieved by creating a static class, which is helpful in specific cases. As a general practice, it's best to avoid using global variables because they violate the object-oriented programming philosophy of C# and can make things complicated while multi-threading, etc. Be cautious to avoid conflicts by adding locks or ensuring that only one thread has access to the global variable at any given instance.

The scope of a variable is within the full code block of its declaration. Also, since the code blocks are sometimes nested as per the application requirement, a loop defined within the method of a class gives three nested code blocks and subsequently three levels of nested-scope. If a variable is defined in any of these scopes, it will be visible to the current scope and the ones that are nested within it.

In simple terms:

  • A variable declared within a loop will not be visible outside the loop.
  • A variable that has been declared outside a loop will be accessible from inside the loop too.

Variable Scopes in C#

To understand the scope of a local variable well, it is important to understand the levels of scope in C# .

Class-Level Scope

Variables defined in the class are available to all non-static methods declared in the class, called fields or class members. Their access modifier does not affect their scope within the class, and they can be accessed outside of the class using access modifiers.

For example:

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using System; 
class ClassScope { // class level scope starts here
    int abc = 1000; // class level variable with class level scope 
    public void display() { 
    	Console.WriteLine(a); // method to access the class level variable 
	} // method ends here
} // class level scope ends 
csharp

Method-Level Scope

Variables declared within a method are available to its corresponding parts and also the nested code blocks. They are not available outside the method. These are local variables. Variables cease to exist after method execution is complete.

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static void Main(string[] args) {
    int marks;                                          // Declared at the method-level
    marks = 100;                                        // Used at the method-level
	if (marks >= 50)
    	Console.WriteLine("Great Marks are : {0}", marks);   // Used in the nested scope
	else
    	Console.WriteLine("Bad marks are : {0}", marks);   // Again, used in the nested scope
}
csharp

Output will be:

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Great Marks are : 100

Nested Scope

We have explained earlier that variables declared in a nested scope will not be available outside their respective code blocks. These can be called loop variables. We will show in the following example that a variable declared in a nested scope of an if statement cannot be used at the method level and will not compile.

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static void Main(string [] args){
    int score = 100;
    if (score >= 60)
        string message = "Good score";                  // Declared in if statement 
    else
    	string message = "Poor score";                  // Declared in else statement
    Console.WriteLine(message);                         // Variable unavailable
}
csharp

Output will be:

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Error(s):
(15:9) Embedded statement cannot be a declaration or labeled statement
(17:6) Embedded statement cannot be a declaration or labeled statement
console

If we wanted to make this code work, we could declare the variable before the if statement and assign a value to it within the if statement.

*Note: There is a crucial difference in between the scope definition in C and C#.

If a C# variable is defined within the local scope in a block (if/else) which is conflicting with a variable defined outside following that block, it will give an error. A similar code is compiled under C/C++ or Java. Local variables will remain in scope throughout the entire block where they have been declared. This is the opposite of C++, where the local variables are in scope at points in their block only after they have been declared.

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public void function(){
  if (true) {
    /* scope of local if */
    int a = 2;
    System.Console.WriteLine(a);
  } else {
    /* no conflict arises with same if/else */
    int a = 4;
    System.Console.WriteLine(a);
  }

  if (true) {
    /* no conflict with local from different if scope */
    int a = 10;
    System.Console.WriteLine(a);
  }
}
csharp

We know that while declaring a scope, any local variable from the outer scope is known. There is no possibility that a local variable within a scope would override the local variable from an outside scope.

Scopes and Related Error Messages

The general rules of thumb for C# are:

  • It is erroneous for a local variable declaration and nested local variable declaration space to contain elements of same name.
  • There will occur a compile time error if, within the scope of a local variable, it is in a textual position that occurs before the local-variable-declarator. If the local variable declaration is implicit, there will still be an error to refer to that variable in its local-variable-declarator.

These rules can be understood and remembered with a simple example.

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class Clarity {
    public int time;

    void Function() {
        int seconds;
        seconds = 0; // (Line 1) Will bind to local variable defined above
        time = 0; // (Line 2) Binds to field time

        {
            seconds = "s"; // (Line 3) Will bind to local variable defined below
            string seconds;

            time = "s"; // (Line 4) Binds to local variable defined below.
            string time;
        }
    }
}
csharp

At line 2, one would think that seconds would bind to Clarity.seconds just as in line 1. However, the C# spec defines clearly that name will resolve to closest scope. At line 3 and line 4 there will be compiler errors.

However, a reference to the local variable inside the declarator is allowed, though it is not implicitly typed. So the following will be true:

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int a = (x = 5); // Allowed
var b = (y = 10); // Will result in an error
csharp

In the first statement, we have already declared the type to be int. When the binding takes place, it is okay because the left side (variable initializer) is apt. Meanwhile, we did not have a type for b initially, so when the binding takes place, we are not certain if 10 can even be assigned to b or not.

One thing that can be done is name hiding. This is allowed only on fields that are not referenced in current scope. We are allowed to redefine a variable as a string because it has not been referenced yet in the scope for a given method.

We realize that this method of using a name before its declarator causes the compiler to generate errors, which is not ideal. In simple terms, the plain name is resolved to whatever statement is declared inside its current block, regardless of whether there exists a same name variable or definition outside the block.

Also, in the 2005 C# Compiler, we incorrectly bound both statements of s to outer local variable, and line 1 would bind perfectly. But there was an error on line 2 reporting that the string is not convertible to int. There was also an error on line 3 that you cannot redeclare s to something else.

Then, in the 2008 C# Compiler, this was fixed to correctly reflect the spec. Both line 1 and line 2 would bind to line 3. Since they were textually before their declaration, both would yield an error, saying that they were being used before being declared. Line 3 also gives an error that you cannot redeclare s.

Conclusion

This guide has explained the scope of local variables in C#. Armed with this knowledge, we can avoid common binding issues and errors and define variables in the correct scope. If an unknown error or garbage value occurs within variables, it can be resolved by assigning values once the scope is well known.

Happy learning C#!

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