C# referencing a variable from another method

Usually you'd pass it as an argument, like so:

void Method1()
{
    var myString = "help";
    Method2(myString);
}

void Method2(string aString)
{
    var myString = "I need ";
    var anotherString = myString + aString;
}

However, the methods in your example are event listeners. You generally don't call them directly. (I suppose you can, but I've never found an instance where one should.) So in this particular case it would be more prudent to store the value in a common location within the class for the two methods to use. Something like this:

string StringA { get; set; }

public void button1_Click(object sender, EventArgs e)
{ 
   StringA = "help";
}

public void button2_Click(object sender, EventArgs e)
{
    string b = "I need ";
    string c = b + StringA;
}

Note, however, that this will behave very differently in ASP.NET. So if that's what you're using then you'll probably want to take it a step further. The reason it behaves differently is because the server-side is "stateless." So each button click coming from the client is going to result in an entirely new instance of the class. So having set that class-level member in the first button click event handler won't be reflected when using it in the second button click event handler.

In that case, you'll want to look into persisting state within a web application. Options include:

1. Page Values (hidden fields, for example)
2. Cookies
3. Session Variables
4. Application Variables
5. A Database
6. A Server-Side File
7. Some other means of persisting data on the server side, etc.

You need to declare string a in the scope of the class, not the method, at the moment it is a "local variable".

Example:

private string a = string.Empty;

public void button1_Click(object sender, EventArgs e) 
{  
    a = "help"; 
} 

public void button2_Click(object sender, EventArgs e) 
{ 
    //this is where I need to call the string "a" value from button1_click  
    string b = "I need"; 
    string c = b + a;           
} 

You can now access the value of your "private field" a from anywhere inside your class which in your example will be a Form.

Agree with @Devid 's answer but I prefer to create a class of required entities and then use them in entire solution without passing variable as argument.

Classname.variableName;

for ex-

 Class argumentData{
public static string firstArg= string.Empty;
public static string secArg= string.Empty;
}

Say I am assigning data in function

void assignData()
{
argumentData.firstArg="hey";
argumentData.secArg="hello";
}

if I want to use it in another method then

void showData()
{
Console.WriteLine("first argument"+argumentData.firstArg);
Console.WriteLine("sec argument"+ argumentData.secArg);
}

Hope this helps!