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How to Resolve Error "CS0136: A local variable named 'var' cannot be declared in this scope" in C#

The Compiler Error CS0136 is a Variable Shadowing error. The message reads: "A local variable named 'var' cannot be declared in this scope because it would give a different meaning to 'var', which is already used in a 'parent or current/child' scope to denote something else".

C# enforces a strict scoping rule: A variable declared in an inner block (child scope) cannot have the same name as a variable declared in an enclosing block (parent scope).

Unlike C++, where an inner variable "hides" the outer one, C# considers this ambiguous and error-prone. The compiler prevents you from accidentally modifying a local variable when you meant to modify the outer one (or vice versa).

Understanding Block Scope​

In C#, scope is generally defined by curly braces { }.

  • Parent Scope: The method body.
  • Child Scope: Blocks inside the method, such as if, for, foreach, or using blocks.

Variables defined in the Parent are visible to the Child. Therefore, the Child cannot define a new variable with the same name, because the name is already taken.

note

C# vs. C++: In C++, declaring a variable inside an inner block "shadows" (hides) the outer variable. C# explicitly forbids this to prevent logic bugs where developers confuse which variable they are accessing.

Scenario 1: Redeclaration Inside if or loops​

This is the most common occurrence. You define a variable at the start of your method, and then accidentally try to define it again inside an if statement.

Example of error​

public void ProcessTransaction(bool isConfirmed)
{
    int status = 0; // Declared in Parent Scope

    if (isConfirmed)
    {
        // ⛔️ Error CS0136: A local variable named 'status' cannot be declared in this scope
        // 'status' is already defined in the parent scope above.
        int status = 1; 
        
        Console.WriteLine(status);
    }
}

Solution: Reuse or Rename​

Option A: Reuse (Assign) If you meant to update the existing variable, remove the int keyword.

public void ProcessTransaction(bool isConfirmed)
{
    int status = 0;

    if (isConfirmed)
    {
        // ✅ Correct: Updates the outer variable
        status = 1; 
    }
}

Option B: Rename (Distinct Logic) If the inner variable is logically distinct, give it a new name.

public void ProcessTransaction(bool isConfirmed)
{
    int status = 0;

    if (isConfirmed)
    {
        // ✅ Correct: 'innerStatus' is a unique name
        int innerStatus = 1; 
        Console.WriteLine(innerStatus);
    }
}

Scenario 2: Nested Loops (The 'i' Conflict)​

When copying and pasting loops, it is easy to forget to change the iterator variable (usually i).

Example of error​

public void PrintMatrix(int[,] matrix)
{
    for (int i = 0; i < 10; i++) // Outer loop uses 'i'
    {
        // ⛔️ Error CS0136: 'i' cannot be declared in this scope
        // The outer 'i' is still visible here.
        for (int i = 0; i < 10; i++) 
        {
            Console.WriteLine(matrix[i, i]);
        }
    }
}

Solution: Use Different Iterators​

Standard convention is to use j, k, etc., for nested loops.

public void PrintMatrix(int[,] matrix)
{
    for (int i = 0; i < 10; i++)
    {
        // ✅ Correct: Using 'j' avoids the collision
        for (int j = 0; j < 10; j++)
        {
            Console.WriteLine(matrix[i, j]);
        }
    }
}

Scenario 3: Lambda Expressions and LINQ​

This error frequently appears when using Lambda expressions (=>). The parameters of a lambda expression are local variables within the lambda's scope. If a parameter name conflicts with a local variable outside the lambda, CS0136 is triggered.

Example of error​

using System;
using System.Linq;
using System.Collections.Generic;

public class Data
{
    public void FilterData()
    {
        int id = 5; // Local variable

        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

        // ⛔️ Error CS0136: The lambda parameter 'id' conflicts with the local variable 'id'.
        var result = numbers.Where(id => id > 0);
    }
}

Solution: Rename Lambda Parameter​

Change the parameter name in the lambda expression to something generic (like x, n, or item) or specific (num).

public void FilterData()
{
    int id = 5; 

    List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

    // ✅ Correct: 'n' does not conflict with 'id'
    var result = numbers.Where(n => n > 0);
}

Conclusion​

CS0136 is a safety feature of the C# compiler.

  1. Check Inner Blocks: Look inside your if, foreach, or while blocks. Are you declaring a variable (e.g., string name) that already exists outside?
  2. Check Iterators: Ensure nested loops use different variables (i, j, k).
  3. Check Lambdas: Ensure your lambda parameters (x => ...) don't share names with variables defined right above them.

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