What does new() in method name means C# syntax

Question

I was reading a code of library and I saw the following syntax. I have searched quiet a lot on the Google to find out the syntax name but I haven't found anything. Any help would be appreciated.

        /// <summary>
        /// Returns a singleton object that is used to manage the creation and
        /// execution of setup
        /// </summary>
        /// <typeparam name="TMvxSetupSingleton">The platform specific setup singleton type</typeparam>
        /// <returns>A platform specific setup singleton</returns>
        protected static TMvxSetupSingleton EnsureSingletonAvailable<TMvxSetupSingleton>()
           where TMvxSetupSingleton : MvxSetupSingleton, new()
        {
            // Double null - check before creating the setup singleton object
            if (Instance != null)
                return Instance as TMvxSetupSingleton;
            lock (LockObject)
            {
                if (Instance != null)
                    return Instance as TMvxSetupSingleton;

                // Go ahead and create the setup singleton, and then
                // create the setup instance. 
                // Note that the Instance property is set within the 
                // singleton constructor
                var instance = new TMvxSetupSingleton();
                instance.CreateSetup();
                return Instance as TMvxSetupSingleton;
            }
        }

Please notice , new () {. What does it means?

score 2 · Answer 1 · answered Jan 08 '20 at 13:24

According to the MSDN:

The new constraint specifies that a type argument in a generic class declaration must have a public parameterless constructor. To use the new constraint, the type cannot be abstract.

Apply the new constraint to a type parameter when a generic class creates new instances of the type, as shown in the following example:

class ItemFactory<T> where T : new()
{
    public T GetNewItem()
    {
        return new T();
    }
}

When you use the new() constraint with other constraints, it must be specified last:

public class ItemFactory2<T>
    where T : IComparable, new()
{  }

More informations here

score 1 · Accepted Answer · answered Jan 08 '20 at 13:22

From the Microsoft Docs

The where clause may also include a constructor constraint, new(). That constraint makes it possible to create an instance of a type parameter using the new operator. The new() Constraint lets the compiler know that any type argument supplied must have an accessible parameterless constructor. For example:

public class MyGenericClass<T> where T : IComparable<T>, new()
{
    // The following line is not possible without new() constraint:
    T item = new T();
}

The new() constraint appears last in the where clause. The new() constraint can't be combined with the struct or unmanaged constraints. All types satisfying those constraints must have an accessible parameterless constructor, making the new() constraint redundant.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/where-generic-type-constraint

What does new() in method name means C# syntax

2 Answers2