C# WPF Declare an ICommand in a single line

Question

I'm in the process of learning WPF and the MVVM design pattern. Currently the code in my ViewModel for a delete customer command looks like this:

    public class vmCustomers : INotifyPropertyChanged
    {
...
        private ICommand _commandDeleteCustomer = null;
...
        public ICommand CommandDeleteCustomer
        {
            get
            {
                if (_commandDeleteCustomer == null)
                    _commandDeleteCustomer = new RelayCommand<object>(DeleteCustomerAction, DeleteCustomerPredicate);
                return _commandDeleteCustomer;
            }
        }

        private void DeleteCustomerAction(object o)
        {
            ...stuff...
        }

        private bool DeleteCustomerPredicate(object o)
        {
            ...stuff...
            return true;
        }
    }

I'd like to slim down the declaration of the ICommand to something like this so that I can reduce the coding overhead for each command:

public readonly ICommand CommandDeleteCustomer = new RelayCommand((obj) => DeleteCustomerAction(obj), (obj) => DeleteCustomerPredicate(obj));

But I get this error:

A field initializer cannot reference the non-static field, method, or property vmCustomers.DeleteCustomerAction(object)

Is there a way I can declare the ICommand in a single line of code so that I can simply focus on business-related code rather than repeated infrastructure code.

Answer 1

In case your RelayCommand.CanExecuteChanegd hooks onto the CommandManager.RequerySuggested event you can drop the constructor initialization and initialize the property as read-only computed property in a single line statement:

// Creates a *new instance* on every Get() access
public ICommand CommandDeleteCustomer => new RelayCommand(...);

// The following is the verbose form of the previous single line property declaration:
public ICommand CommandDeleteCustomer
{
  get => new RelayCommand(...);
}

To ensure that the same instance is returned from the property, use

a) the null coalescing assignment operator (C# 8.0)

// Backing field required 
private ICommand commandDeleteCustomer;
public ICommand CommandDeleteCustomer => this.commandDeleteCustomer ??= new RelayCommand(...);

// The following is the verbose form of the previous property declaration
// and uses the null coalescing operator:
public ICommand CommandDeleteCustomer => this.commandDeleteCustomer = this.commandDeleteCustomer ?? new RelayCommand(...);

b) a property initializer (single line statement):

// Returns the *same instance* on every Get() access.
// Note that you can't reference instance members like fields and properties from the initializer. 
// This means the command delegates would have to be defined as 'private static'.
public ICommand CommandDeleteCustomer { get; } = new RelayCommand<object>(DeleteCustomerAction, DeleteCustomerPredicate);

private static void DeleteCustomerAction(object o)
{
  ...stuff...
}

private static bool DeleteCustomerPredicate(object o)
{
  ...stuff...
  return true;
}