Is a string property itself threadsafe?

public String SampleProperty{
    get;
    private set;
}

private string sampleField;
private object threadSafer = new object();

public String SampleProperty{
    get{ return this.sampleField; }
    private set{
        lock(threadSafer){
            sampleField = value;
        }
    }
 }

class BackgroundTaskDemo
{
    private bool stopping = false;

    static void Main()
    {
        BackgroundTaskDemo demo = new BackgroundTaskDemo();
        new Thread(demo.DoWork).Start();
        Thread.Sleep(5000);
        demo.stopping = true;
    }

    static void DoWork()
    {
         while (!stopping)
         {
               // Do something here
         }
    }
}

lock(someExternalLock) {
    record.Foo = "Bar";
}

lock(record.SyncLock) {
    record.Foo = "Bar";
}

obj.SampleProperty = "Dear " + firstName + " " + lastName;

obj.SampleProperty += obj.SampleProperty + "a";

obj.SampleProperty = "Initial String Value";