How do I Parameterize a null string with DBNull.Value clearly and quickly

问题

I got tired of writing the following code:

/* Commenting out irrelevant parts
public string MiddleName;
public void Save(){
    SqlCommand = new SqlCommand();
    // blah blah...boring INSERT statement with params etc go here. */
    if(MiddleName==null){
        myCmd.Parameters.Add("@MiddleName", DBNull.Value);
    }
    else{
        myCmd.Parameters.Add("@MiddleName", MiddleName);
    }
    /*
    // more boring code to save to DB.
}*/

So, I wrote this:

public static object DBNullValueorStringIfNotNull(string value)
{
    object o;
    if (value == null)
    {
        o = DBNull.Value;
    }
    else
    {
        o = value;
    }
    return o;
}

// which would be called like:
myCmd.Parameters.Add("@MiddleName", DBNullValueorStringIfNotNull(MiddleName));

If this is a good way to go about doing this then what would you suggest as the method name? DBNullValueorStringIfNotNull is a bit verbose and confusing.

I'm also open to ways to alleviate this problem entirely. I'd LOVE to do this:

myCmd.Parameters.Add("@MiddleName", MiddleName==null ? DBNull.Value : MiddleName);

but that won't work because the "Operator '??' cannot be applied to operands of type 'string and 'System.DBNull'".

I've got C# 3.5 and SQL Server 2005 at my disposal if it matters.

回答1:

Cast either of your values to object and it will compile.

myCmd.Parameters.Add("@MiddleName", MiddleName==null ? (object)DBNull.Value : MiddleName);

回答2:

You can avoid the explicit cast to object using SqlString.Null instead of DBNull.Value:

MiddleName ?? SqlString.Null

There are corresponding types for int, datetime, and so forth. Here's a code snippet with a couple more examples:

 cmd.Parameters.AddWithValue("@StartDate", StartDate ?? SqlDateTime.Null);
 cmd.Parameters.AddWithValue("@EndDate", EndDate ?? SqlDateTime.Null);
 cmd.Parameters.AddWithValue("@Month", Month ?? SqlInt16.Null);
 cmd.Parameters.AddWithValue("@FormatID", FormatID ?? SqlInt32.Null);
 cmd.Parameters.AddWithValue("@Email", Email ?? SqlString.Null);
 cmd.Parameters.AddWithValue("@ZIP", ZIP ?? SqlBoolean.Null);

回答3:

Personally this is what I would do with an extension method (make sure this goes into a static class)

public static object GetStringOrDBNull(this string obj)
{
    return string.IsNullOrEmpty(obj) ? DBNull.Value : (object) obj
}

Then you'd have

myCmd.Parameters.Add("@MiddleName", MiddleName.GetStringOrDBNull());

回答4:

myCmd.Parameters.Add("@MiddleName", MiddleName ?? (object)DBNull.Value);

回答5:

@David Thanks for your suggestion. The following method works great!

MiddleName ?? (object)DBNull.Value

回答6:

Yeap, we'd all love to do myCmd.Parameters.Add("@MiddleName", MiddleName ?? DBNull.Value);. Or better still, have the freakin' SqlClient layer understand that CLR null should be mapped to DBNull.Value when adding a parameter. Unfortunately the .Net type system closes the first alternative, and the implementation of SqlClient closes the second.

I'd go with a well known function name, like Coalesce or IsNull. Any DB developer will recognize what they do in an instant, from the name alone.

回答7:

I'd rather give you two totally different suggestions:

Use an ORM. There are plenty of non-intrusive ORM tools.

Write your own wrapper for building commands, with a cleaner interface. Something like:

public class MyCommandRunner {
  private SqlCommand cmd;

  public MyCommandRunner(string commandText) {
    cmd = new SqlCommand(commandText);
  }

  public void AddParameter(string name, string value) {
    if (value == null)
     cmd.Parameters.Add(name, DBNull.Value);
    else
      cmd.Parameters.Add(name, value);
  }

  // ... more AddParameter overloads
}

If you rename your AddParameter methods to just Add, you can use it in a very slick way:

var cmd = new MyCommand("INSERT ...")
  {
    { "@Param1", null },
    { "@Param2", p2 }
  };

回答8:

I would suggest using nullable properties instead of public fields and an 'AddParameter' method (don't know if this code is optimized or correct, just off the top of my head):


private string m_MiddleName;

public string MiddleName
{
  get { return m_MiddleName; }
  set { m_MiddleName = value; }
}

.
.
.

public static void AddParameter(SQLCommand cmd, string parameterName, SQLDataType dataType, object value)
{
  SQLParameter param = cmd.Parameters.Add(parameterName, dataType);

  if (value is string) { // include other non-nullable datatypes
    if (value == null) {
      param.value = DBNull.Value;
    } else {
      param.value = value;
    }
  } else { 

    // nullable data types
    // UPDATE: HasValue is for nullable, not object type
    if (value.HasValue) // {{{=====================================================
    {
          param.value = value;
    } else 
    {
          param.value = DBNull.Value;
    }
  }
}

.
.
.
AddParameter(cmd, "@MiddleName", SqlDbType.VarChar, MiddleName);

来源：https://stackoverflow.com/questions/2602949/how-do-i-parameterize-a-null-string-with-dbnull-value-clearly-and-quickly

标签

.net

sql-server

naming-conventions