nullable on the models?

Question

What is nullable for in a model? For example:

public int? ProjectId;

If I put it this way I’m accepting null values, right?

And if I put the required Annotation in the property, being required it would have no value to be null or would have?

Why would anyone use this? if there is the possibility of the value being null why not just leave without the ?

Answer 1

I like to think of Dataannotation`s validation in the models as being "messages" intended for the end user, which indicate what it is necessary to do in order for the entered data to be considered correct.

When a model has a property whose type is a struct nullable (such as Int32?, DateTime?, Double?, among others) and at the same time has the annotation Required in one of these properties, it is as if the program says so to the user:

Mr user, this model is only valid when the P property is filled. You can even fail to fill the value of it (after all it is nullable), but in this case I (the program) I won’t save anything and I’ll show an error message.

For me, all the DataAnnotations of validation serve to restrict the possibilities of the type of data used, ie make invalid possible values for the type of data.

In my view, the attribute Required should only be applied to properties whose data type can be overridden. That is, the attribute serves to make invalid the possibility that the data type gives.

Answering your questions more directly:

• Q: And if I put the required Annotation in the property, being required it would have no value to be null or would have?

  • R: when the property is required the value can still be null... occurs when the user leaves the field blank, in which case the ModelState will indicate an error in the template.

• Q: Why would anyone use this? if there is the possibility of the value being null why not just leave without the ?

  • R: If the property type is int (or any other struct) and without Required, this automatically implies a mandatory field... it’s as if the Required were there. In this example, if the user leaves the field blank, the value associated with the model will be the int pattern: default(int) that is 0, whereas the ModelState indicate that the property contains an error as it has not been filled in.

Answer 2

[...] if there is the possibility of the value being null because it does not only leave without the ?

A Nullable Type allows the possibility of a nonvalue (Null in this case) for a Value Type (or type of value), since these by default always have a value - so the first part of this question is not correct.

The following value types have versions Nullable:

Tipo de Valor         Valor Padrão
bool                  false
byte                  0
char                  '\0'
decimal               0.0M
double                0.0D
enum                  O primeiro valor (retornado pela expressão (E)0)
float                 0.0F
int                   0
long                  0L
sbyte                 0
short                 0
uint                  0
ulong                 0
ushort                0

Reference Types (or reference types) may by definition not have a reference, thus not referring to this resource.

Further reading:

Value Types Default Values
http://msdn.microsoft.com/en-us/library/83fhsxwc.aspx

Using Nullable Types
http://msdn.microsoft.com/en-us/library/vstudio/2cf62fcy(v=vs.100). aspx

Types of Value and Reference
http://msdn.microsoft.com/en-us/library/t63sy5hs.aspx

Answer 3

Exactly, the nullable will be so that you do not use the default values of each type.

Now the Dataanottation you will use for a part control.