var vs dynamic

1.var
Type is decided at compile time
Strongly typed
Must be initialized at declaration
IntelliSense & compile-time checking available
Example
var x = 10; // int
var name = "Ram"; // string
👉 If you assign a wrong type later → compile-time error

---

2.dynamic
Type is decided at runtime
Not strongly typed
No compile-time checking
Errors occur at runtime
Example
dynamic x = 10;
x = "Ram"; // allowed
x.DoSomething(); // runtime error if method not found

---

3.Key Differences (Interview Points)

Feature	var	dynamic
Type resolution	Compile time	Runtime
Type safety	Yes	No
IntelliSense	Yes	No
Error checking	Compile time	Runtime
Type change allowed	No	Yes

---

4.When to use?
Use var when type is obvious and you want clean code
Use dynamic when working with:
COM objects
Reflection
JSON / loosely typed data

---

5.Important interview one-liner

var is statically typed, dynamic is dynamically typed.

abstract class

1. What is an Abstract Class?
An abstract class is a partially implemented class.
It can have 
It can have both abstract methods (no body) and normal methods (with body).

2. Key rule
An abstract class cannot be instantiated
It must be inherited by another class

3. What can an abstract class contain?
Abstract methods
Non-abstract (normal) methods
Fields
Properties
Constructors

4. Syntax example (interview-friendly)
abstract class Employee
{
public abstract void Work();

public void Login()
{
// common code
}
}

class Developer : Employee
{
public override void Work()
{
// implementation
}
}

5. Method implementation rule
Abstract methods must be overridden in the derived class
Normal methods are optional to override

6. Inheritance rule
Supports single inheritance only
Multiple inheritance is not allowed with classes

7. Abstract class vs Interface (short point)
Abstract class → Partial abstraction
Interface → Complete abstraction

8. Why do we use abstract classes?
To share common base functionality
To enforce base rules with flexibility
When some behavior is common and some is different

9. Important interview one-liner

An abstract class provides a base class with both implemented and unimplemented methods.

Interface - An interface defines what to do, not how to do it.

1. What is an Interface?
An interface is a contract.
It defines 
It defines what methods or properties a class must have, but not the implementation.

2. What can an interface contain?
Method declarations
Properties
Events
👉 No method body (before C# 8.0)

3. How do we use an interface?
A class implements an interface
The class must implement all methods of the interface

4. Syntax example (interview-friendly)
interface IEmployee
{
void Work();
}

class Developer : IEmployee
{
public void Work()
{
// implementation
}
}

5. Multiple inheritance support
C# does not support multiple inheritance with classes
But a class can implement multiple interfaces
class MyClass : IInterface1, IInterface2
{
}

6. Interface vs Abstract Class (short point)
Interface → 100% abstraction
Abstract class → Partial abstraction

7. Why do we use interfaces?
Loose coupling
Dependency Injection
Multiple inheritance
Better maintainability and testability

8. Important interview one-liner

An interface defines what to do, not how to do it.

How to Find Repeated Values in Excel Using COUNTIF

To identify repeated values in Excel,

after adding a new column for count,

after typing =COUNTIF(, come to the left arrow and move to the required column. After that, press Ctrl + Shift + End to go till the end. Then, put a comma. Next, press Ctrl + Shift + Up Arrow to come to the beginning of the column. After coming there, click Enter, then the result will come. If you drag the result, the output will come for all columns.

=COUNTIF(A2:A248,A2) - example 

How to Clear Visual Studio Cache || Visual Studio Component Cache Location

Visual Studio Component Cache Location
Main Path (User-specific)
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio
Inside this folder, you will see version-specific folders like:
16.0_xxxxxx → Visual Studio 2019
17.0_xxxxxx → Visual Studio 2022

---

Common Cache Folders You Can Clear
ComponentModelCache (Most important)
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio\16.0_xxxxxx\ComponentModelCache
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio\17.0_xxxxxx\ComponentModelCache
Clearing this fixes:
Visual Studio slowness
IntelliSense issues
Extension load errors

---

MEF Cache
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio\16.0_xxxxxx\Cache

---

Roslyn Cache (C# / IntelliSense)
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio\17.0_xxxxxx\Roslyn

---

Designer Shadow Cache
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio\Designer

---

How to Clear Visual Studio Cache (Safe Steps)
Method 1: Manual (Recommended)
Close Visual Studio completely
Press Win + R
Paste:
%USERPROFILE%\AppData\Local\Microsoft\VisualStudio
Open the required version folder (16.0 / 17.0)
Delete these folders:
ComponentModelCache
Cache
Roslyn (optional)
Restart Visual Studio
Cache will be recreated automatically

---

Method 2: Command Prompt
devenv /clearcache
Run from Developer Command Prompt for Visual Studio

---

What NOT to Delete
Do NOT delete:
Extensions folder
Settings files
.vs folder inside your solution (unless troubleshooting)

If you want the list of databases currently being used / active connections

 SELECT 
    DB_NAME(database_id) AS DatabaseName,
    COUNT(*) AS ActiveConnections
FROM sys.dm_exec_sessions
WHERE database_id > 0
GROUP BY database_id
ORDER BY ActiveConnections DESC;

If you want BOTH list of customer DBs + active connections
SELECT 
    d.name AS DatabaseName,
    ISNULL(c.ActiveConnections, 0) AS ActiveConnections
FROM sys.databases d
LEFT JOIN (
    SELECT database_id, COUNT(*) AS ActiveConnections
    FROM sys.dm_exec_sessions
    GROUP BY database_id
) c ON d.database_id = c.database_id

WHERE d.name LIKE 'C%';

Could not copy "obj\Debug\etc" to "obj\Debug\etc". Exceeded retry count of 10. Failed.

 1. Stop the Running Application
Make sure your app (yourapp(ur application name)) is not running.
Open Task Manager (Ctrl+Shift+Esc) → go to Details tab.
Find yourapp(ur application name)) → End Task.
Rebuild your project.

2. Clean and Rebuild
In Visual Studio:
Go to Build → Clean Solution
Then Build → Rebuild Solution
This clears the obj and bin folders and rebuilds everything fresh.

3. Check for Debugger/Antivirus Locks
Sometimes, even if the EXE isn’t running:
Antivirus software may be scanning or locking the file — temporarily disable real-time protection or add your project folder toa exceptions.
Visual Studio Debugger might not have released the file — close the solution and reopen it.

4. Manually Delete Locked Files
If the above doesn’t work:
Close Visual Studio.
Open File Explorer → navigate to your project folder.
Delete the folders:
bin\Debug
obj\DebugReopen Visual Studio → Rebuild.
If Windows says “file is in use,” use a tool like Process Explorer:
Download from Microsoft Sysinternals.Search for (yourapp(ur application name)) → right-click → Kill Process.

5. Disable Parallel Builds (Optional)
If your solution has multiple projects:
Go to Tools → Options → Projects and Solutions → Build and RunSet “maximum number of parallel project builds” = 1
This avoids concurrent file access issues.

6. Check Post-Build Events

If you have a post-build script that copies or runs the EXE, ensure it’s not launching the app before the next build.

How to download (clone) a Git repository into a specific folder ?

If you want to download (clone) a Git repository into a specific folder, you can do it very easily using the git clone command with a folder path at the end. 

Option 1 – Clone directly into a specific folder Command format: git clone 

Example: git clone https://github.com/username/Notes.git D:\Projects\NotesApp 

This will: Download the Git repository into the folder D:\Projects\NotesApp If the folder doesn’t exist, Git will create it automatically. 

Option 2 – Clone in current folder with custom name. If you’re in a directory (e.g. D:\Projects) and you want to clone the repo into a subfolder with a custom name, do this: 

cd D:\Projects (Enter)

git clone https://github.com/username/Notes.git 

MyNotes It will create a folder called MyNotes inside D:\Projects and put the repo code there.

Get Location

Your Location

Getting location...

XML declaration.

<?xml version="1.0" encoding="utf-8"?> 

This is the XML declaration.

version="1.0": Tells the XML parser to use version 1.0 rules.

encoding="utf-8": Says the text is encoded in UTF-8 (supports all languages and symbols).