How to refer to strings in an external class (C#)

If I right understood your question, you can do something like this:

public class2 
{
    public static string MyString 
    {
       get {return "foobar"; }
    }

}


public class1 
{
    public void DoSomething() 
    {
       MessageBox.Show(class2.MyString );
    }
}

Something like this?

public static class Foo
{
    public const string FOO_CONST = "value";
}

public class Bar
{
    public void Bar()
    {
        Console.WriteLine(Foo.FOO_CONST);
    }
}

If you create a new instance of Class2 you can make MyString public or pull it out into a get method:

//In Class1
Class2 class2 = new Class2();
MessageBox.Show(class2.Mystring());

//In Class2
public string Mystring{ get; set; }

Or you could return the string from the method

public static string myMethod()
{
    string myString = "foobar";
    //logic goes here
    return myString;
}

//In Class1
Class2 class2 = new Class2();
MessageBox.Show(class2.MyMethod());

Based on your clarification to your question:

I am trying to check a boolean value in a method in class2. E.g. if the method run in class2 changes the boolean value in that method, the method in class1 can check this and do some logic

You could do something like this:

class Class1 {

  Class2 myClass = new Class2();

  public void ActivityMethod() {
    myClass.MethodThatMayChangeBoolean();
    if(myClass.myBoolean) {
      // Response to a truth change.
    } else {
      // Respond to a false change.
    }
  }
}

class Class2 {
  public boolean myBoolean { get; }

  public void MethodThatMayChangeBoolean() {
    // Do stuff in here that may change boolean.
  }
}

You will need to use Properties

 private static string _mystring = "foobar";
 public static string mystring 
 {
    get { return _mystring ; }
    set { _mystring = value; }
 }

Or use auto properties and initialize their values in the static constructor of the class:

 public static string mystring { get; set; }

 public static MyStaticClass()
 {
   mystring = "foobar";
 }