Is it possible to coalesce string and DBNull in C#?

问题

I'm writing a C# routine to call a stored proc. In the parameter list I'm passing in, it is possible that one of the values can legally be null. So I thought I'd use a line like this:

cmd.Parameters.Add(new SqlParameter("@theParam", theParam ?? DBNull.Value));

Unfortunately, this returns the following error:

CS0019: Operator '??' cannot be applied to operands of type 'string' and 'System.DBNull'

Now, this seems clear enough, but I don't understand the rationale behind it. Why would this not work? (And often, when I don't understand why something isn't working, it's not that it can't work...it's that I'm doing it wrong.)

Do I really have to stretch this out into a longer if-then statement?

EDIT: (As an aside, to those suggesting to just use "null" as is, it doesn't work. I originally figured null would auto-translated into DBNull too, but it apparently does not. (Who knew?))

回答1:

Not like that, no. The types have to match. The same is true for the ternary.

Now, by "match", I don't mean they have to be the same. But they do have to be assignment compatible. Basically: in the same inheritance tree.

One way to get around this is to cast your string to object:

var result = (object)stringVar ?? DBNull.Value;

But I don't like this, because it means you're relying more on the SqlParameter constructor to get your types right. Instead, I like to do it like this:

cmd.Parameters.Add("@theParam", SqlDbTypes.VarChar, 50).Value = theParam;
// ... assign other parameters as well, don't worry about nulls yet

// all parameters assigned: check for any nulls
foreach (var p in cmd.Parameters) 
{ 
    if (p.Value == null) p.Value = DBNull.Value; 
}

Note also that I explicitly declared the parameter type.

回答2:

new SqlParameter("@theParam", (object)theParam ?? DBNull.Value)

回答3:

The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand. But in your case they are different types, so it doesn't work.

回答4:

The Null Coalesce operator only with with data of the same type. You cannot send NULL to the SqlParamater as this will make Sql Server says that you didn't specify the parameter.

You can use

new SqlParameter("@theParam", (object)theParam ?? (object)DBNull.Value)

Or you could create a function that return DBNull when null is found, like

public static object GetDataValue(object o)
{
    if (o == null || String.Empty.Equals(o))
        return DBNull.Value;
    else
        return o;
}

And then call

new SqlParameter("@theParam", GetDataValue(theParam))

回答5:

The reason you can't use the null coalesce operator is that it has to return one type and you are providing more than one type. theParam is a string. DbNull.Value is a reference to a static instance of type System.DbNull. This is what its implementation looks like;

public static readonly DBNull Value = new DBNull(); 
//the instantiation is actually in the 
//static constructor but that isn't important for this example

So if you were to have a NullCoalesce method, what would its return type be? It can't be both System.String and System.DbNull, it has to be one or the other, or a common parent type.

So that leads to this type of code;

cmd.Parameters.Add(
    new SqlParameter("@theParam", (object)theParam ?? (object)DBNull.Value)
);

回答6:

In your stored proc when you declare the incoming variable, have it set the var equal to null and then do not pass it in from your csharp code, it will then pick up the default value from sql

@theParam as varchar(50) = null

and then in your csharp

if (theParam != null)
    cmd.Parameters.Add(new SqlParameter("@theParam", theParam));

This is how I usually pass option and/or defaulted values to my stored procs

回答7:

I'm pretty sure that just passing a null to the SqlParameter constructor results in it being sent as a DBNull.Value... I may be mistaken, since I use the EnterpriseLibraries for DB access, but I'm quite sure that sending a null is fine there.

回答8:

cmd.Parameters.Add(new SqlParameter("@theParam", (theParam == null) ? DBNull.Value : theParam));

回答9:

Use this syntax:

(theParam as object) ?? (DBNull.Value as object)

In this case both parts of operator ?? are of the same type.

回答10:

Not sure the specific answer to your question, but how about this?

string.IsNullOrEmpty(theParam) ? DBNull.Value : theParam

or if blank is ok

(theParam == null) ? DBNull.Value : theParam

来源：https://stackoverflow.com/questions/1545711/is-it-possible-to-coalesce-string-and-dbnull-in-c

标签

null-coalescing-operator