How to Give a C# Auto-Property an Initial Value

What is the best way to give a C# auto-property an initial value?

In C# 5 and earlier, to give auto implemented properties an initial value, you have to do it in a constructor.

Since C# 6.0, you can specify initial value in-line. The syntax is:

public int X { get; set; } = x; // C# 6 or higher

DefaultValueAttribute is intended to be used by the VS designer (or any other consumer) to specify a default value, not an initial value. (Even if in designed object, initial value is the default value).

At compile time DefaultValueAttribute will not impact the generated IL and it will not be read to initialize the property to that value (see DefaultValue attribute is not working with my Auto Property).

Example of attributes that impact the IL are ThreadStaticAttribute, CallerMemberNameAttribute, ...

How do you give a C# Auto-Property a default value When the Property is a Class

Since C#6 you can initialize auto-implemented properties:

public Person Member { get; set; } = new Person { PersonTypeID = 1 }; // or by using the constructor of Person
public Person Claimant { get; set; } = new Person { PersonTypeID = 2 };

otherwise use the constructor of Claim.

How to set default value for Auto-Implemented Properties in ASP.NET

You can initialize the property in the default constructor:

public MyClass()
{
   IsPopup = true;
}

With C# 6.0 it is possible to initialize the property at the declaration like normal member fields:

public bool IsPopup { get; set; } = true;  // property initializer

It is now even possible to create a real read-only automatic property which you can either initialize directly or in the constructor, but not set in other methods of the class.

public bool IsPopup { get; } = true;  // read-only property with initializer

Auto-property default value

Yes, but to be able to create an instance from outside your class, make your constructor public.

public class MyClass
{
    public MyClass()
    {
        Reason = "my reason";
    }

    public string Reason {get; set; }
}

Set a default value to a property

No, there is no built-in way to set the value of a property with metadata. You could use a factory of some sort that would build instances of a class with reflection and then that could set the default values. But in short, you need to use the constructors (or field setters, which are lifted to the constructor) to set the default values.

If you have several overloads for your constructor, you may want to look at constructor chaining.

Using C# 6+, you are able to do something like this...

public string MyValue { get; set; } = "My Default";

Oh, it gets more fun because people have even requested something like this...

// this code won't compile!
public string MyValue {
    private string _myValue;
    get { return _myValue ?? "My Default"; }
    set { _myValue = value; }
}

... the advantage being that you could control the scope of the field to only be accesible in the property code so you don't have to worry about anything else in your class playing with the state without using the getter/setter.

Set default value for auto-implemented property

You're almost there; you just have to initialize the value in the constructor:

public class MyClass
{
    public MyClass()
    {
        Foo = 1;
    }

    public int Foo { get; set; }
}

How to return a default value for a property?

If you are using C# 6+, it has added the ability to assign a default value to auto-properties. So you can simply write something like this:

public string Name { get; set; } = "No Name";
public DateTime Created { get; private set; } = DateTime.Today;

This will set the default value of the Name property to No Name and the Created property to today's date, as you wanted.

automatic property with default value

If the default value for the type is not sufficient, then the only way to do it is via a constructor.

How to get default value of auto property in C# using reflection?

That class is actually compiled into this:

public class MyClass
{
    private int MyClass___1<Id> = 5;
    public int Id
    {
        get { return MyClass___1<Id>; }
        set { MyClass___1<Id> = value; }
    }
}

Which in turn is equivalent to this:

public class MyClass
{
    private int MyClass___1<Id>;

    public MyClass()
    {
        MyClass___1<Id> = 5;
    }

    public int Id
    {
        get { return MyClass___1<Id>; }
        set { MyClass___1<Id> = value; }
    }
}

So other than either decompiling the constructor, or constructing an instance of the type, you can't get that value.

If you're the author of the type, the best way to do this would be to apply the [DefaultValue(...)] attribute to the property, which can be read through reflection. Lots of serialization libraries will use this value as well, to avoid serializing a property that has the default value.

Your class would then look like this:

public class MyClass
{
    [DefaultValue(5)]
    public int Id { get; set; } = 5;
}

Having said that, here's a small LINQPad program that demonstrates Mono.Cecil:

void Main()
{
    var assembly = AssemblyDefinition.ReadAssembly(GetType().Assembly.Location);
    var myClassType =
        (from module in assembly.Modules
         from type in module.Types
         where type.Name == "UserQuery"
         from nestedType in type.NestedTypes
         where nestedType.Name == "MyClass"
         select nestedType).FirstOrDefault();

    var ctor =
        (from method in myClassType.Methods
         where method.IsConstructor
         select method).FirstOrDefault();

    foreach (var instruction in ctor.Body.Instructions)
        Console.WriteLine(instruction.ToString());
}

public class MyClass
{
    public int Id { get; set; } = 5;
}

Output:

IL_0000: ldarg.0
IL_0001: ldc.i4.5
IL_0002: stfld System.Int32 UserQuery/MyClass::<Id>k__BackingField
IL_0007: ldarg.0
IL_0008: call System.Void System.Object::.ctor()
IL_000d: ret

So it might be feasible in this case, but other types of constants, and so on, like calls to static methods, would not be easy to handle.

In short, create the instance!