C＃，Nullable十进制

Question

In my database, v1 field is nullable field. but I define 0 (default) if the value is null.

public decimal? v1 {
  get {
    return this._v1; 
  }
  set {
    this._v1 = value ?? 0M;
  }
}

so now, the v1 is not nullable variable anymore.

but I can not do this,

decimal v2 = v1;

The error message say, Cannot implicitly convert type 'decimal?' to 'decimal'.

In this case, Do I have to convert to decimal, like this?

decimal v2 = Convert.ToDecimal(v1);

It very annoying job. and codes are look dirty too.

Anyone know better solution? please advice me.

Answer 1

No you don't have to convert the decimal? , you have access to the underlying value from the Nullable type eg

decimal v2 = v1.Value;

Assigning a default value to a Nullable type does not make it non-nullable, it just means it has a value. Nullable types have a HasValue property which helps you determine this.

Just for the record, I wouldn't recommend defaulting the value to 0 it would probably make more sense letting it default to null considering it can indeed be null. If you need to have a default value in your app you will probably want to use the GetValueOrDefault method eg

decimal v2 = v1.GetValueOrDefault(0m);

Answer 2

The easiest way to convert a decimal? to decimal but have the value to be 0 when null , you can do:

decimal? a = null;
decimal b = a.GetValueOrDefault(0m); // will contain 0 when null, otherwise the value

Read up on GetValueOrDefault

You could also call GetValueOrDefault with no arguments, which will yield default(T) , which in the case of Decimal is 0, but I like to be explicit.

Answer 3

I'm curious why you would define your app code variable as nullable but then set it to 0 if it's null. Is it just to avoid the exception if passing a null value from the database to your app variable? If your application doesn't absolutely need the nullable decimal variable, just check it for a DbNull as you're reading it out:

decimal myAppVal = rdr["DbColumn"] == DBNull.Value 
   ? default(decimal) 
   : (decimal) rdr["DbColumn"];

or explicitly set 0.0M instead of using default(decimal)

Design-wise, if you're finding yourself doing this often, then i'd suggest creating a generic extension method to read values out of the database. This way you can ensure that invalid casts are avoided and exceptions are handled, but more importantly that default values are returned in case of nulls. ;)

Something like (and I'm just spitballing here):

public static T CastFromDbTo<T>(object readerObject)
{
    T returnVal = default(T);
    if (readerObject is T)
    {
        var myValue = (T) readerObject;
        returnVal = readerObject != DbNull.Value && myValue != null
                ? (T) readerObject 
                : default(T);
    }
    return returnVal;
}

Then you could grab your value like this:

var myAppValue = HelperClass.CastFromDbTo<decimal>(rdr["DbColumn"]);

or actually make an extension:

public static T CastFromDbTo<T>(this object readerObject)
{
    T returnVal = default(T);
    if (readerObject is T)
    {
        var myValue = (T) readerObject;
        returnVal = readerObject != DbNull.Value && myValue != null
                ? (T) readerObject 
                : default(T);
    }
    return returnVal;
}

then you could just do this:

var myAppVal = rdr["DbColumn"].CastFromDbTo<decimal>();

Answer 4

The Value you assigned to v1 is decimal , but the type is still decimal? . That's why you can't assing it.

Try

decimal v2 = v1.Value;

Answer 5

Nullable Types have the properties Value and HasValue .

HasValue - Gets a value indicating whether the current Nullable object has a value.

Value - If HasValue is true, this contains the value if not its null.

decimal v2;

if (v1.HasValue) // check for null
{
  v2 = v1.Value;
}

Check out MSDN - Nullable Types (C# Programming Guide)