Problem with SqlParameterCollection.Add method to make SqlDbType all NVarChar

Question

I am trying to access a SQL Server table with a program created in C# to update and register data.

I am thinking of parameterizing the query and using the SqlParameterCollection.Add method to create a SQL statement.

In this case, if I set SqlDbtype to NVarChar for both Name and Age, as in the example below, is there any problem?

query ="INSERT INTO [myTable] ([Name],[Age]) VALUES (@Name,@Age)";
using(SqlCommand cmd = new SqlCommand(query, conn)) 
{
  cmd.Parameters.Add(new SqlParameter("@Name", SqlDbType.NVarChar));
  cmd.Parameters.Add(new SqlParameter("@Age", SqlDbType.NVarChar));
  cmd.Parameters["@Name"].Value = "Tom";
　cmd.Parameters["@Age"].Value = "28";　
  cmd.ExecuteNonQuery(); 
}

In the program I am trying to create, not all of the corresponding fields are of type NVARCHAR.

Is there any problem, such as an error that would occur if I set the SqlDbtype to NVarChar, even when inserting into an Int or DateTime2 type?

Answer 1

First, I agree with everyone else, that the normal procedure is that the data type you specify match the type in the database.

In this particular case, what could happen if you pass the value 'Twentyeight" is that you get a conversion error from SQL Server. If you specify the correct data type, you will instead get the conversion error on .NET level. You could argue that is more or less the same.

However, there are other situations. Say that the data type in SQL Server is date or datetime and you specify the parameter as NVarChar rather than Date or DateTime. This means that SQL Server will convert the string to date rather than the client. Say that the string is 05/02/2025. Say that the user's regional settings is English (United Kingdom). Then this will be interpreted as 5 February 2025. But in the SQL Server, it is quite likely that the user setting is us_english, because this is the default, and the string will be interpreted as 2 May 2025. You always want date and time strings to be interpreted in the client, according to the user's regional settings.

Another situation is this:

SELECT ... FROM tbl WHERE varcharcol = @x

If you pass @x as NVarChar, SQL Server will apply its data-precedence rules and convert varcharcol to nvarchar. If there is an index on varcharcol, this will have an effect on performance. If the collation is a Windows collation, the index can still be used as intended, but with some overhead, expect a factor of 2 or 3. If the collation is an SQL collation, the index is not usable, and there will be a scan, and for a large table this can lead to a performance disaster.

Answer 2

Hi, @那由多. Welcome to Microsoft Q&A. 

SqlDbType should be specified as the type of the corresponding field in the database.

For example: If the Age field in the database is of type Int, then SqlDbType should be SqlDbType.Int.

string query = "INSERT INTO myTable (Name,Age) VALUES (@Name,@Age)";
using (SqlCommand cmd = new SqlCommand(query, conn))
{
    cmd.Parameters.Add(new SqlParameter("@Name", SqlDbType.NVarChar));
    cmd.Parameters.Add(new SqlParameter("@Age", SqlDbType.Int));
    cmd.Parameters["@Name"].Value = "Tom";
    cmd.Parameters["@Age"].Value = 28;
    cmd.ExecuteNonQuery();
}
conn.Close();

If the database field is not of NVarChar type, using SqlDbType.NVarChar cannot guarantee that the conversion will be successful. Sometimes a conversion failure error will occur, making the program less robust.

You could compare the document to specify the correct SqlDbType type.

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Answer 3

Is there any problem, such as an error that would occur if I set the SqlDbtype to NVarChar, even when inserting into an Int or DateTime2 type?

It does not make sense to me to consider such irregular settings unless you have very special and concrete reasons to do so. Please explain your reason.

I suggest that you read the following Microsoft document. Match the "SQL Server Database Engine type" with ".NET Framework type" in your ADO.NET code and select proper "SqlDbType enumeration" when you create SqlParameter object

SQL Server Data Type Mappings

Answer 4

I would recommend using NuGet package Dapper.

For example, note there is no need to open/close the connection; parameters are done for you.

using System;
using System.Data;
using System.Data.SqlClient;
using Dapper;

class Program
{
    private static string _connectionString = "TODO";

    static void Main()
    {
        using var conn = new SqlConnection(_connectionString);

        string query = "INSERT INTO [myTable] ([Name],[Age]) VALUES (@Name, @Age)";

        var parameters = new { Name = "Tom", Age = 28 };

        conn.Execute(query, parameters);
        
        Console.WriteLine("Record inserted successfully.");
    }
}

When not using Dapper, each parameter created needs to match the underlying column type.

Answer 5

with command parameters you are defining the datatype of the parameter when sent to sqlserver. if the parameter type does not match its use a conversion is required.

there are implicit data type conversion, used when a one value is set to another and explicit conversions done with cast or convert function.

if the datatype of the parameter can be implicitly converted to data type required then it will work. but a string value can not be converted to a number for instance. see implicit list:

https://learn.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql?view=sql-server-ver16

varchar and and nvarchar can implicitly convert. but an encoding choice must be made. as all c# string are unicode (nvarchar), you should always use SqlDbType.NVarChar for a string parameter, as the default encoding in your application may not match the encoding in the SQL Server.