Export (0) Print
Expand All

Configuring Parameters and Parameter Data Types

Command objects use parameters to pass values to SQL statements or stored procedures, providing type checking and validation. Unlike command text, parameter input is treated as a literal value, not as executable code. This helps guard against "SQL injection" attacks, in which an attacker inserts a command that compromises security on the server into an SQL statement.

Parameterized commands can also improve query execution performance, because they help the database server accurately match the incoming command with a proper cached query plan. For more information, see Execution Plan Caching and Reuse and Parameters and Execution Plan Reuse in SQL Server Books Online. In addition to the security and performance benefits, parameterized commands provide a convenient method for organizing values passed to a data source.

A DbParameter object can be created by using its constructor, or by adding it to the DbParameterCollection by calling the Add method of the DbParameterCollection collection. The Add method will take as input either constructor arguments or an existing parameter object, depending on the data provider.

When adding parameters, you must supply a ParameterDirection property for parameters other than input parameters. The following table shows the ParameterDirection values that you can use with the ParameterDirection enumeration.

Member name

Description

Input

The parameter is an input parameter. This is the default.

InputOutput

The parameter can perform both input and output.

Output

The parameter is an output parameter.

ReturnValue

The parameter represents a return value from an operation such as a stored procedure, built-in function, or user-defined function.

The syntax for parameter placeholders depends on the data source. The .NET Framework data providers handle naming and specifying parameters and parameter placeholders differently. This syntax is customized to a specific data source, as described in the following table.

Data provider

Parameter naming syntax

System.Data.SqlClient

Uses named parameters in the format @parametername.

System.Data.OleDb

Uses positional parameter markers indicated by a question mark (?).

System.Data.Odbc

Uses positional parameter markers indicated by a question mark (?).

System.Data.OracleClient

Uses named parameters in the format :parmname (or parmname).

The data type of a parameter is specific to the .NET Framework data provider. Specifying the type converts the value of the Parameter to the .NET Framework data provider type before passing the value to the data source. You may also specify the type of a Parameter in a generic manner by setting the DbType property of the Parameter object to a particular DbType.

The .NET Framework data provider type of a Parameter object is inferred from the .NET Framework type of the Value of the Parameter object, or from the DbType of the Parameter object. The following table shows the inferred Parameter type based on the object passed as the Parameter value or the specified DbType.

.NET Framework type

DbType

SqlDbType

OleDbType

OdbcType

OracleType

Boolean

Boolean

Bit

Boolean

Bit

Byte

Byte

Byte

TinyInt

UnsignedTinyInt

TinyInt

Byte

byte[]

Binary

VarBinary. This implicit conversion will fail if the byte array is larger than the maximum size of a VarBinary, which is 8000 bytes. For byte arrays larger than 8000 bytes, explicitly set the SqlDbType.

VarBinary

Binary

Raw

Char

 

Inferring a SqlDbType from char is not supported.

Char

Char

Byte

DateTime

DateTime

DateTime

DBTimeStamp

DateTime

DateTime

DateTimeOffset

DateTimeOffset

DateTimeOffset in SQL Server 2008. Inferring a SqlDbType from DateTimeOffset is not supported in versions of SQL Server earlier than SQL Server 2008.

DateTime

Decimal

Decimal

Decimal

Decimal

Numeric

Number

Double

Double

Float

Double

Double

Double

Single

Single

Real

Single

Real

Float

Guid

Guid

UniqueIdentifier

Guid

UniqueIdentifier

Raw

Int16

Int16

SmallInt

SmallInt

SmallInt

Int16

Int32

Int32

Int

Int

Int

Int32

Int64

Int64

BigInt

BigInt

BigInt

Number

Object

Object

Variant

Variant

Inferring an OdbcType from Object is not supported.

Blob

String

String

NVarChar. This implicit conversion will fail if the string is larger than the maximum size of an NVarChar, which is 4000 characters. For strings larger than 4000 characters, explicitly set the SqlDbType.

VarWChar

NVarChar

NVarChar

TimeSpan

Time

Time in SQL Server 2008. Inferring a SqlDbType from TimeSpan is not supported in versions of SQL Server earlier than SQL Server 2008.

DBTime

Time

DateTime

UInt16

UInt16

Inferring a SqlDbType from UInt16 is not supported.

UnsignedSmallInt

Int

UInt16

UInt32

UInt32

Inferring a SqlDbType from UInt32 is not supported.

UnsignedInt

BigInt

UInt32

UInt64

UInt64

Inferring a SqlDbType from UInt64 is not supported.

UnsignedBigInt

Numeric

Number

 

AnsiString

VarChar

VarChar

VarChar

VarChar

 

AnsiStringFixedLength

Char

Char

Char

Char

 

Currency

Money

Currency

Inferring an OdbcType from Currency is not supported.

Number

 

Date

Date in SQL Server 2008. Inferring a SqlDbType from Date is not supported in versions of SQL Server earlier than SQL Server 2008.

DBDate

Date

DateTime

 

SByte

Inferring a SqlDbType from SByte is not supported.

TinyInt

Inferring an OdbcType from SByte is not supported.

SByte

 

StringFixedLength

NChar

WChar

NChar

NChar

 

Time

Time in SQL Server 2008. Inferring a SqlDbType from Time is not supported in versions of SQL Server earlier than SQL Server 2008.

DBTime

Time

DateTime

 

VarNumeric

Inferring a SqlDbType from VarNumeric is not supported.

VarNumeric

Inferring an OdbcType from VarNumeric is not supported.

Number

user-defined type (an object with SqlUserDefinedAggregateAttribute

Object or String, depending the provider (SqlClient always returns an Object, Odbc always returns a String, and the OleDb managed data provider can see either

SqlDbType.Udt if SqlUserDefinedTypeAttribute is present, otherwise Variant

OleDbType.VarWChar (if value is null) otherwise OleDbType.Variant.

OdbcType.NVarChar

not supported

Note Note

Conversions from decimal to other types are narrowing conversions that round the decimal value to the nearest integer value toward zero. If the result of the conversion is not representable in the destination type, an OverflowException is thrown.

Note Note

When you send a null parameter value to the server, you must specify DBNull, not null (Nothing in Visual Basic). The null value in the system is an empty object that has no value. DBNull is used to represent null values. For more information about database nulls, see Handling Null Values.

Parameters can also be derived from a stored procedure using the DbCommandBuilder class. Both the SqlCommandBuilder and OleDbCommandBuilder classes provide a static method, DeriveParameters, which automatically populates the parameters collection of a command object that uses parameter information from a stored procedure. Note that DeriveParameters overwrites any existing parameter information for the command.

Note Note

Deriving parameter information incurs a performance penalty because it requires an additional round trip to the data source to retrieve the information. If parameter information is known at design time, you can improve the performance of your application by setting the parameters explicitly.

For more information, see Generating Commands with CommandBuilders.

Stored procedures offer many advantages in data-driven applications. By using stored procedures, database operations can be encapsulated in a single command, optimized for best performance, and enhanced with additional security. Although a stored procedure can be called by passing the stored procedure name followed by parameter arguments as an SQL statement, by using the Parameters collection of the ADO.NET DbCommand object enables you to more explicitly define stored procedure parameters, and to access output parameters and return values.

Note Note

Parameterized statements are executed on the server by using sp_executesql, which allows for query plan reuse. Local cursors or variables in the sp_executesql batch are not visible to the batch that calls sp_executesql. Changes in database context last only to the end of the sp_executesql statement. For more information, see SQL Server Books Online.

When using parameters with a SqlCommand to execute a SQL Server stored procedure, the names of the parameters added to the Parameters collection must match the names of the parameter markers in the stored procedure. The .NET Framework Data Provider for SQL Server does not support the question mark (?) placeholder for passing parameters to an SQL statement or a stored procedure. It treats parameters in the stored procedure as named parameters and searches for matching parameter markers. For example, the CustOrderHist stored procedure is defined by using a parameter named @CustomerID. When your code executes the stored procedure, it must also use a parameter named @CustomerID.

CREATE PROCEDURE dbo.CustOrderHist @CustomerID varchar(5)

This example demonstrates how to call a SQL Server stored procedure in the Northwind sample database. The name of the stored procedure is dbo.SalesByCategory and it has an input parameter named @CategoryName with a data type of nvarchar(15). The code creates a new SqlConnection inside a using block so that the connection is disposed when the procedure ends. The SqlCommand and SqlParameter objects are created, and their properties set. A SqlDataReader executes the SqlCommand and returns the result set from the stored procedure, displaying the output in the console window.

NoteNote

Instead of creating SqlCommand and SqlParameter objects and then setting properties in separate statements, you can instead elect to use one of the overloaded constructors to set multiple properties in a single statement.

static void GetSalesByCategory(string connectionString, 
    string categoryName)
{
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        // Create the command and set its properties.
        SqlCommand command = new SqlCommand();
        command.Connection = connection;
        command.CommandText = "SalesByCategory";
        command.CommandType = CommandType.StoredProcedure;

        // Add the input parameter and set its properties.
        SqlParameter parameter = new SqlParameter();
        parameter.ParameterName = "@CategoryName";
        parameter.SqlDbType = SqlDbType.NVarChar;
        parameter.Direction = ParameterDirection.Input;
        parameter.Value = categoryName;

        // Add the parameter to the Parameters collection. 
        command.Parameters.Add(parameter);

        // Open the connection and execute the reader.
        connection.Open();
        SqlDataReader reader = command.ExecuteReader();

        if (reader.HasRows)
        {
            while (reader.Read())
            {
                Console.WriteLine("{0}: {1:C}", reader[0], reader[1]);
            }
        }
        else
        {
            Console.WriteLine("No rows found.");
        }
        reader.Close();
    }
}

When using parameters with an OleDbCommand or OdbcCommand, the order of the parameters added to the Parameters collection must match the order of the parameters defined in your stored procedure. The .NET Framework Data Provider for OLE DB and .NET Framework Data Provider for ODBC treat parameters in a stored procedure as placeholders and apply parameter values in order. In addition, return value parameters must be the first parameters added to the Parameters collection.

The .NET Framework Data Provider for OLE DB and .NET Framework Data Provider for ODBC do not support named parameters for passing parameters to an SQL statement or a stored procedure. In this case, you must use the question mark (?) placeholder, as in the following example.

SELECT * FROM Customers WHERE CustomerID = ?

As a result, the order in which Parameter objects are added to the Parameters collection must directly correspond to the position of the ? placeholder for the parameter.

OleDbCommand command = new OleDbCommand("SampleProc", connection);
command.CommandType = CommandType.StoredProcedure;

OleDbParameter parameter = command.Parameters.Add(
  "RETURN_VALUE", OleDbType.Integer);
parameter.Direction = ParameterDirection.ReturnValue;

parameter = command.Parameters.Add(
  "@InputParm", OleDbType.VarChar, 12);
parameter.Value = "Sample Value";

parameter = command.Parameters.Add(
  "@OutputParm", OleDbType.VarChar, 28);
parameter.Direction = ParameterDirection.Output;

OdbcCommand command = new OdbcCommand( _
  "{ ? = CALL SampleProc(?, ?) }", connection);
command.CommandType = CommandType.StoredProcedure;

OdbcParameter parameter = command.Parameters.Add( _
  "RETURN_VALUE", OdbcType.Int);
parameter.Direction = ParameterDirection.ReturnValue;

parameter = command.Parameters.Add( _
  "@InputParm", OdbcType.VarChar, 12);
parameter.Value = "Sample Value";

parameter = command.Parameters.Add( _
  "@OutputParm", OdbcType.VarChar, 28);
parameter.Direction = ParameterDirection.Output;
Show:
© 2014 Microsoft