How to make a property required in C#?

Question

I have requirement in a custom class where I want to make one of my properties required.

How can I make the following property required?

public string DocumentType
{
    get
    {
        return _documentType;
    }
    set
    {
        _documentType = value;
    }
}

Answer 1

.NET 7 or newer (c# lang 11+)

Syntax

public class MyClass
{
    public required string Name { get; init; }
}

new MyClass(); // illegal
new MyClass { Name = "Me" }; // works fine

Remarks

The required properties must declare a setter (either init or set).

Access modifiers on properties or setters cannot be less visible than their containing type, as they would make impossible to initialize the class in some cases.

public class MyClass
{
    internal required string Name { get; set; } // illegal
}

Documentation

• Official documentation here
• Feature demo here

.NET 6 or older

See this answer

Answer 2

Records make this very easy. Simply declare the properties of the record and know they'll always be set upon construction. Sometimes we are required to use classes. Even if you create constructors with the required parameters it is still a possibility that you yourself forget to initialize the properties. Even a property that is { get; } may still be uninitialized in case of int or other numerical types (they'll just be left at 0). It is an easy fix, of course, but if you want to avoid even this little mistake you can create an inner record of the class with the required properties and use it in place of the constructor parameters. Like so:

public class MyClass
{
  public MyClass(PropsType props)
  {
    Props = props;
  }

  public PropsType Props { get; }

  public record PropsType(int SomeInt, string SomeString);
}

With this construction you're now pretty much guaranteed to have completely initialized instances of MyClass.

Answer 3

This one liner works in C# 9:

public record Document(string DocumentType);

new Document(); // compiler error
new Document("csv"); // correct way to construct with required parameter

This explains how it works. In the above code, Document is the name of the class or "record". That first line of code actually defines an entire class. In addition to this solution essentially making a required DocumentType property (required by an auto implemented constructor), because it uses records, there are additional implications. So this may not always be an appropriate solution, and the C# 11 required keyword will still come in handy at times. Just using record types doesn't automatically make properties required. The above code is a special syntax way of using records that essentially has this effect as well as making the property init only and causes a deconstructor to be automatically implemented.

Keep in mind a string can be set to empty and that would still satisfy the "required" aspect of this property. Unfortunately I don't know of any good way to do extra validation within the record to make sure the string is not empty or an int is in range, etc. You would have to go deeper down the TOP (type driven development) rabbit hole, which may not be a bad thing. You could create your own type that doesn't allow empty strings or integers outside your accepted range. Unfortunately such an approach would lead to runtime discovery of invalid input instead of compile time. There might be a better way using static analysis and metadata, but I've been away from C# for too long to know anything about that.

Answer 4

The required modifier forces you to set a property or a filed to a value when you instantiate.

public class Movie
{
    public required string Name { get; set; }
    public required string Genre { get; set; }
    public decimal Gross { get; set; }
}

When you attempt to instantiate a Movie without setting the Name and Genre, you will see a compiler error, as shown below:

Required member 'Movie.Name' must be set in the object initializer or attribute constructor. [RequiredModifier]
Required member 'Movie.Genre' must be set in the object initializer or attribute constructor. [RequiredModifier]

As the error says, we have two options to overcome the error.

1. Set their value in the object initializer:

Movie lordOfWar = new()
{
    Name = "Lord of War",
    Genre = "Drama",
    Gross = 72617068M
};

Console.WriteLine("{0} is a {1} film. It was grossed {2:C}",
    lordOfWar.Name, lordOfWar.Genre, lordOfWar.Gross);

2. When you try to set required properties and fields in a constructor, you will still encounter the error. To handle that error, you can import the namespace and then decorate the constructor with the attribute that informs the compiler that all required properties and fields are being set, as shown in the following code:

using System.Diagnostics.CodeAnalysis; // [SetsRequiredMembers]
public class Movie
{
    public required string Name { get; set; }
    public required string Genre { get; set; }
    public decimal Gross { get; set; }

    [SetsRequiredMembers]
    public Movie(string name, string genre)
    {
        Name = name;
        Genre = genre;
    }
}

In the Program.cs:

Movie lordOfWar = new(name: "Lord Of War", genre: "drama")
{
    Gross = 72617068
};

Console.WriteLine("{0} is a {1} film. It was grossed {2:C}.",
    lordOfWar.Name, lordOfWar.Genre, lordOfWar.Gross);

Further information:

Required members

Required modifier

Answer 5

With the release of .NET 7 and C# 11 in November 2022 you can now use the required modifier this way:

public class Person
{
    public Person() { }

    [SetsRequiredMembers]
    public Person(string firstName) => FirstName = firstName;

    public required string FirstName { get; init; } 

    public int Age { get; set; }

}

And when you don't have the required properties it will throw an error when you try to initialize an object.

For more information refer to:

• https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11#required-members

• https://learn.microsoft.com/en-us/dotnet/csharp/properties#init-only

Answer 6

Add a required attribute to the property

Required(ErrorMessage = "DocumentTypeis required.")]
public string DocumentType
        {
            get
            {
                return _documentType;
            }
            set
            {
                _documentType = value;
            }
        }

For custom attribute detail Click Here

Answer 7

I used an other solution, not exactly what you want, but worked for me fine because I declare the object first and based on specific situation I have different values. I didnt want to use the constructor because I then had to use dummy data.

My solution was to create Private Sets on the class (public get) and you can only set the values on the object by methods. For example:

public void SetObject(string mandatory, string mandatory2, string optional = "", string optional2 = "") 

Answer 8

If you mean you want it always to have been given a value by the client code, then your best bet is to require it as a parameter in the constructor:

class SomeClass
{
    private string _documentType;

    public string DocumentType
    {
        get
        {
            return _documentType;
        }
        set
        {
            _documentType = value;
        }
    }

    public SomeClass(string documentType)
    {
        DocumentType = documentType;
    }
}

You can do your validation – if you need it – either in the property's set accessor body or in the constructor.

Answer 9

If you mean "the user must specify a value", then force it via the constructor:

public YourType(string documentType) {
    DocumentType = documentType; // TODO validation; can it be null? blank?
}
public string DocumentType {get;private set;}

Now you can't create an instance without specifying the document type, and it can't be removed after that time. You could also allow the set but validate:

public YourType(string documentType) {
    DocumentType = documentType;
}
private string documentType;
public string DocumentType {
    get { return documentType; }
    set {
        // TODO: validate
        documentType = value;
    }
}