How can I make it so my field is only set the one time?

Question

I have a field in my model class that's coded as follows:

    public string CreatedBy
    {
        get { return _CreatedBy; }
        set { _CreatedBy = value; Created = DateTime.Now; }
    }
    public DateTime? Created { get; set; }

When the CreatedBy field is populated then it fills out the Created date automatically. The problem for me is that if I again set the CreatedBy field (yes it can happen) then the date gets updated with the current date again.

Is there a way that I can make it so the CreatedBy and the Created fields can be populated just once?

Answer 1

您不能在集合中检查是否已经存在一个值，而根本不设置新值？

Answer 2

Make use of constructor and iniailize the property to defult value you want

public classConstructor()
{
   propertyName = defaultValue;
}

Answer 3

Use a backing field and check whether the value has been set already - if so, leave it unchanged:

private DateTime? created;
public DateTime? Created 
{
    get { return created; }
    set { if (created == null) created = value; }
}

Answer 4

Here's a quick way: use the ?? operator. If Created is null, it will go to DateTime.Now.

public string CreatedBy
{
    get { return _CreatedBy; }
    set { _CreatedBy = value; Created = Created ?? DateTime.Now; }
}
public DateTime? Created { get; set; }

Answer 5

In that case it would probably be best to include CreatedBy in the constructor. This would imply "create-time" semantics:

public string CreatedBy
{
    get;
    private set;
}

public DateTime? Created
{
    get;
    private set;
}

public Model(..., string createdBy)
{
    this.CreatedBy = createdBy;
    this.Created = DateTime.Now;
}

// another option, if you don't like the ctor route
public void AssignCreator(string createdBy)
{
    if (this.Created.HasValue) throw new InvalidOperationException();
    this.CreatedBy = createdBy;
    this.Created = DateTime.Now;
}

Your other option would be to throw an InvalidOperationException in the property setter if Created is non-null.