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Error while passing DBNull to SqlParameter

I am trying to pass null value to SqlParameter if the value of variable vName is null, else pass vName value as shown below
cmd.Parameters.Add(new SqlParameter("#NAME", SqlDbType.VarChar)).Value = vName ? null : DBNull.Value;
but I get an error
Cannot implicitly convert type 'string' to 'bool'
I searched and found that I have to use AgeItem.AgeIndex but I got error that says
The name 'AgeItem' does not exist in the current context
cmd.Parameters.Add(new SqlParameter("#NAME", SqlDbType.VarChar)).Value =(object)AgeItem.AgeIndex ?? DBNull.Value;

vName is a string but you use it like it was a bool: vName ? .... Use vName == null ? ...:
cmd.Parameters.Add(new SqlParameter("#NAME", SqlDbType.VarChar)).Value =
vName == null ? DBNull.Value : (Object)vName;

IMHO much cleaner:
if (vName != null)
cmd.Parameters.AddWithValue("#NAME",vName);
else
cmd.Parameters.AddWithValue("#Name",null);

Code should be change as follow
cmd.Parameters.Add(new SqlParameter("#NAME", SqlDbType.VarChar)).Value!= null ? vName: DBNull.Value;

Null values are not inserted in Allow Nulls colums in C#

My query returns false when some values are null, but my table allows Null values.
What did I do wrong?
cmd.CommandText ="Insert into BusinessTbl(BName,BAddress,BEmail,BMobile,BPhone,Cat_Id)" +
"values(#bname,#baddress,#bemail,#bmobile,#bphone,#catid)";
cmd.Parameters.AddWithValue("#bname", b.BusinessName);
cmd.Parameters.AddWithValue("#name", b.BusinessAddress);
cmd.Parameters.AddWithValue("#bemail", b.BusinessEmail);
cmd.Parameters.AddWithValue("#bmobile", b.BusinessMobile);
cmd.Parameters.AddWithValue("#bphone", b.BusinessPhone);
cmd.Parameters.AddWithValue("#catid", b.ddlbcategory);
con.ExecuteNonQuery(cmd);
My Table

This is a vexing feature of ADO.NET parameters; basically:
cmd.Parameters.AddWithValue("#bname", ((object)b.BusinessName) ?? DBNull.Value);
cmd.Parameters.AddWithValue("#name", ((object)b.BusinessAddress) ?? DBNull.Value);
// etc
should fix you. If the .Value is null, the parameter isn't sent - it needs to be DBNull.Value. Alternatively, a tool like "Dapper" helps avoid this pain:
con.Execute(#"Insert into BusinessTbl(BName,BAddress,BEmail,BMobile,BPhone,Cat_Id)
values(#bname,#baddress,#bemail,#bmobile,#bphone,#catid)",
new { bname = b.BusinessName, ... , catid = b.ddlbcategory });
(which will parameterize correctly, including the nulls)

Using null while adding SQL parameters

I would like to add some null's to table:
command.CommandText =
"INSERT into clients (Addres, companyID) VALUES (#Addres, #companyID) ; select SCOPE_IDENTITY();";
command.Parameters.AddWithValue("#Addres", null);
command.Parameters.AddWithValue("#companyID", null);
Table dessign allows null's. Why I have this error then?
The parameterized query '(#Addres nvarchar(4000),#companyID nvarchar(4000))INSERT into cl' expects the parameter '#Addres', which was not supplied.

Use DBNull.Value instead.
command.Parameters.AddWithValue("#Addres", DBNull.Value);
command.Parameters.AddWithValue("#companyID", DBNull.Value);

You have to use DBNull class for null values in SQL. Your code will be like this:
command.Parameters.AddWithValue("#Addres", DBNull.Value);
command.Parameters.AddWithValue("#companyID", DBNull.Value);

You can use System.Data.SqlTypes.SqlString.Null
command.Parameters.AddWithValue("#Addres", System.Data.SqlTypes.SqlString.Null);
command.Parameters.AddWithValue("#companyID", System.Data.SqlTypes.SqlString.Null);
Read: Handling Null Values
DBNull.Value can be used for any type as opposed to SqlTypes.
It can be handy to make the code more readable and type-safe, for example:
var addrVal = new System.Data.SqlTypes.SqlString(someAddress);
// ...
if (condition) addrVal = System.Data.SqlTypes.SqlString.Null;
command.Parameters.AddWithValue("#Addres", addrVal);

cmd.Parameters.AddWithValue("#param1",param1 ?? DbNull.Value)

I have int? type value which can be either null or have some value. I want to execute a insert statement using parametrized query to insert accordingly.
int? val1=null;
SqlCeCommand cmd;
string insert = "insert into table(col1)values(#col_1)";
cmd = new SqlCeCommand();
cmd.CommandText = insert;
cmd.Parameters.AddWithValue("#col_1", val1 ?? Convert.ToInt32(DBNull.Value)));
cmd.Connection = cn1;
cn1.Open();
cmd.ExecuteNonQuery();
cn1.Close();
This code throws format exception error. I even tried
cmd.Parameters.AddWithValue("#col_1", (!val1.HasValue) ? val1 : Convert.ToInt32(DBNull.Value));
How shld it b done?

no need to convert if null set DBNull
cmd.Parameters.AddWithValue("#col_1", (object)val1 ??DBNull.Value);

I would believe that changing the line to the following should work:
cmd.Parameters.AddWithValue("#col_1", val1 ?? DBNull.Value);
Since this would set the parameter value to an object type.
If that fails for some reason try to cast the result to an object like this:
cmd.Parameters.AddWithValue("#col_1", (object)(val1 ?? DBNull.Value));
But this should not be necessary.
Edit:
Actually, this should work:
cmd.Parameters.AddWithValue("#col_1", val1 ?? (object)DBNull.Value);

You Should change add parameter code like this:
if (val1.HasValue)
{
cmd.Parameters.AddWithValue("#col_1", val1.Value);
}
else
{
cmd.Parameters.AddWithValue("#col_1", DBNull.Value);
}

SqlParameterCollection only accepts non-null SqlParameter type objects, not DBNull objects

When I add the SQL parameter p to the collection I get an InvalidCastException with the message from the post title.
parentId is a nullable integer and a nullable integer in the database.
Why do I get this exception and how can I solve it?
I do not use stored procedures and I have read the similar threads but they did not help me.
var p = new SqlParameter("ParentId", SqlDbType.Int).Value = parentId ?? (object) DBNull.Value;
cmd.Parameters.Add(p);

You aren't adding your new SqlParameter. p is the result of new SqlParameter("ParentId", SqlDbType.Int).Value = parentId ?? (object) DBNull.Value. In other words, p itself is DBNull.Value.
Split the statement in two, like so:
var p = new SqlParameter("ParentId", SqlDbType.Int);
p.Value = parentId ?? (object) DBNull.Value;
cmd.Parameters.Add(p);
Alternatively,
var p = new SqlParameter("ParentId", SqlDbType.Int) { Value = parentId ?? (object) DBNull.Value };
cmd.Parameters.Add(p);
Either would make sure p is the parameter, not the parameter's value.

You need to use:
System.Data.SqlTypes.SqlInt32.Null