The nameof operator allows you to get the name of a variable, type or member in string form without hard-coding it as a literal. The operation is evaluated at compile-time, which means that you can rename, using an IDE's rename feature, a referenced identifier and the name string will update with it.

var myString = "String Contents";
Console.WriteLine(nameof(myString));

Would output

myString

because the name of the variable is "myString". Refactoring the variable name would change the string.

If called on a reference type, the nameof operator returns the name of the current reference, not the name or type name of the underlying object. For example:

string greeting = "Hello!";
Object mailMessageBody = greeting;

Console.WriteLine(nameof(greeting)); // Returns "greeting"
Console.WriteLine(nameof(mailMessageBody)); // Returns "mailMessageBody", NOT "greeting"!

Snippet

public void DoSomething(int paramValue)
{
    Console.WriteLine(nameof(paramValue));
}

...

int myValue = 10;
DoSomething(myValue);

Console Output

paramValue

Snippet

public class Person : INotifyPropertyChanged
{
    private string _address;

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public string Address
    {
        get { return _address; }
        set
        {
            if (_address == value)
            {
                return;
            }

            _address = value;
            OnPropertyChanged(nameof(Address));
        }
    }
}

...

var person = new Person();
person.PropertyChanged += (s,e) => Console.WriteLine(e.PropertyName);

person.Address = "123 Fake Street";

Console Output

Address

Snippet

public class BugReport : INotifyPropertyChanged
{
    public string Title { ... }
    public BugStatus Status { ... }
}

...

private void BugReport_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    var bugReport = (BugReport)sender;

    switch (e.PropertyName)
    {
        case nameof(bugReport.Title):
            Console.WriteLine("{0} changed to {1}", e.PropertyName, bugReport.Title);
            break;

        case nameof(bugReport.Status):
            Console.WriteLine("{0} changed to {1}", e.PropertyName, bugReport.Status);
            break;
    }
}

...

var report = new BugReport();
report.PropertyChanged += BugReport_PropertyChanged;

report.Title = "Everything is on fire and broken";
report.Status = BugStatus.ShowStopper;

Console Output

Title changed to Everything is on fire and broken

Status changed to ShowStopper

Snippet

public class SomeClass<TItem>
{
    public void PrintTypeName()
    {
        Console.WriteLine(nameof(TItem));
    }
}

...

var myClass = new SomeClass<int>();
myClass.PrintTypeName();

Console.WriteLine(nameof(SomeClass<int>));

Console Output

TItem

SomeClass

Snippet

Console.WriteLine(nameof(CompanyNamespace.MyNamespace));
Console.WriteLine(nameof(MyClass));
Console.WriteLine(nameof(MyClass.MyNestedClass));
Console.WriteLine(nameof(MyNamespace.MyClass.MyNestedClass.MyStaticProperty));

Console Output

MyNamespace

MyClass

MyNestedClass

MyStaticProperty

Prefer

public class Order
{
    public OrderLine AddOrderLine(OrderLine orderLine)
    {
        if (orderLine == null) throw new ArgumentNullException(nameof(orderLine));
        ...
    }
}

Over

public class Order
{
    public OrderLine AddOrderLine(OrderLine orderLine)
    {
        if (orderLine == null) throw new ArgumentNullException("orderLine");
        ...
    }
}    

Using the nameof feature makes it easier to refactor method parameters.

Instead of the usual loosely typed:

@Html.ActionLink("Log in", "UserController", "LogIn")

You can now make action links strongly typed:

@Html.ActionLink("Log in", @typeof(UserController), @nameof(UserController.LogIn))

Now if you want to refactor your code and rename the UserController.LogIn method to UserController.SignIn, you don't need to worry about searching for all string occurrences. The compiler will do the job.