How to insert null into database?

问题

Hi I am trying to insert null in a database column depending on a gridview datakeys value (if being "" insert null into database) However, I am getting a space ' ' inside the database column.

string sbcId = gvTest.DataKeys[gr.RowIndex]["myColumn"].ToString();
 insgnp.Parameters.Add(new OleDbParameter("EMPID", (sbcId==""?DBNull.Value.ToString():sbcId)));

回答1:

You have to rewrite your code:

if(string.IsNullOrEmpty(sbcId))
    Parameters.Add(new OleDbParameter("EMPID", DBNull.Value)); 
else
    Parameters.Add(new OleDbParameter("EMPID", sbcId));

The problem with the ternary if statement that you have is that its returntype must always be the same, which is why you cannot use it (string and DbNull.Value are not compatible)

回答2:

Use DBNull.Value, not DBNull.Value.ToString. (With other parameter collection types in .NET just using null would also work, but I'm not 100% sure about OleDbParameter).

As for the tertiary operator. Don't cast to string, but cast the string to object:

sbcId=="" ? DBNull.Value : (object)sbcId

回答3:

You need to use DBNull.Value not DBNull.Value.ToString()

DBNull.Value.ToString() is ''

This program gives

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("'{0}'", DBNull.Value.ToString());
        }
    }
}

回答4:

Try below by removing ToString() after DBNull.Value

string sbcId = gvTest.DataKeys[gr.RowIndex]["myColumn"].ToString();
 insgnp.Parameters.Add(new OleDbParameter("EMPID", (sbcId==""?DBNull.Value:sbcId)));

回答5:

SQLString.Null should do the job just fine :)

回答6:

If you Insert a non string value in the access database you may write the insert query in this way

Insert into tableName (column1,column2) values (NULL,NULL);

But you want to insert null value in Short Text or Long Text field in access you may write the insert query in this way

Insert into tableName (column1, column2) values('','');

回答7:

I prefer to use an extension method to handle instead of multiple if statements. This allows you to account for null or empty string values. Also allows it to be reusable.

    public static object GetDBNullOrValue<T>(this T val)
    {
        bool isDbNull = true;
        Type t = typeof(T);

        if (Nullable.GetUnderlyingType(t) != null)
            isDbNull = EqualityComparer<T>.Default.Equals(default(T), val);

        else if (t.IsValueType)
            isDbNull = false;

        else if (t == typeof(string) && val != null)
        {
            string temp = val as String;
            isDbNull = (temp == String.Empty);
        }

        else
            isDbNull = (val == null);

        return isDbNull ? DBNull.Value : (object)val;
    }

Then you could use

     Parameters.Add(new OleDbParameter("EMPID", sbcId.GetDBNullOrValue()));

回答8:

your string like that

string value = "";

now add following line

 if (value == ""){

  value = "NULL";
     }

now insert in db and check you have null value their be logical

来源：https://stackoverflow.com/questions/3913241/how-to-insert-null-into-database

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ASP.NET

c#-2.0