C# 4 default parameter values: How to assign a default DateTime/object value? [duplicate]

Question

If DateTime is an object and default C# parameters can only be assigned compile-time constants, how do you provide default values for objects like DateTime?

I am trying to initialize values in a POCO with a constructor, using named parameters with default values.

Dumond

Call an overloaded method that sets the parameter: https://mcmap.net/q/188518/-set-default-value-for-datetime-in-optional-parameter-duplicate – Loose

Answer 1

DateTime cannot be used as a constant but you could make it a nullable type (DateTime?) instead.

Give the DateTime? a default value of null, and if it is set to null at the start of your function, then you can initialize it to any value you want.

static void test(DateTime? dt = null)
{
    if (dt == null)
    {
        dt = new DateTime(1981, 03, 01);
    }

    //...
}

You can call it with a named parameter like this:

test(dt: new DateTime(2010, 03, 01));

And with the default parameter like this:

test();

Pathology

What about DateTime.MinValue? – Dumond

@Dr.Zim: No it's readonly but not constant. – Pathology

And why wouldn't DateTime.MinValue be a compiler-time constant? – Dumond

@Dr. Zim: because DateTime can't be declared as const. I don't know why that's the way it is. – Pathology

you can use: DateTime dt = default(DatetTime) – Ruffian

if (!dt.HasValue) startDate = new DateTime(2010, 03, 01); – Deannadeanne

Answer 2

The only way you can do this directly is to use the value default(DateTime), which is compile-time constant. Or you can work around that by using DateTime? and setting the default value to null.

See also this related question about TimeSpan.

Mitchellmitchem

Just what I was looking for. Sometimes using DateTime? is messier. – Undulate

thanks.. default or minvalue by @Dr. Zim does the trick for me. but default is what i like. saved me from posting another question! :) – Visitant

Answer 3

new DateTime() also equals DateTime.MinValue

You could a create a default parameter like so.

void test(DateTime dt = new DateTime())
{
//...
}

Faroff

Doesn't work, you cannot use functions in a default parameter. – Hydantoin

@Hydantoin it works try it. – Faroff

I can't believe it compiles when I do this. I have yet to see, though, if it will be fine at runtime. – Liturgist

Since it is DateTime is struct it will work. – Catachresis

Answer 4

Unlike VB, C# doesn't support date literals. And since optional parameters look like this in IL, you can't fake it with attributes.

.method private hidebysig static void foo([opt] int32 x) cil managed
{
    .param [1] = int32(5)
    .maxstack 8
    L_0000: nop 
    L_0001: ret 
}



.method //this is a new method
private hidebysig static //it is private, ???, and static
void foo  //it returns nothing (void) and is named Foo
([opt] int32 x) //it has one parameter, which is optional, of type int32

.param [1] = int32(5) //give the first param a default value of 5

Humility

Can you please explain this code? – Oleograph

ok thank you :) Date time also we can give the same manner like .param [1] = DateTime(5) ? – Oleograph

Answer 5

private System.String _Date= "01/01/1900";
public virtual System.String Date
{
   get { return _Date; }
   set { _Date= value; }
}

We can assign value to a label like given below,

lblDate.Text = Date;

Also we can get the value,

DateTime dt = Convert.ToDateTime(label1.Text);

Oleograph

Cannot be used as parameter still. Does not answer the question. – Dustydusza

Answer 6

you could use:

Datetime.MinValue

for initialization.

Jetblack

Doesn't answer the question as you can't set this as the default value as it's not a constant – Rationalism