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Configuring parameters

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Applies to: .NET Framework .NET .NET Standard

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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 behavior helps guard against "SQL injection" attacks, in which an attacker inserts a command that compromises security on the server into a 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 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.

Supply the ParameterDirection property

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 value 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.

Work with parameter placeholders

The syntax for parameter placeholders depends on the data source. The Microsoft SqlClient Data Provider for SQL Server handles naming and specifying parameters and parameter placeholders differently. The SqlClient data provider uses named parameters in the format @parametername.

Specify parameter data types

The data type of a parameter is specific to the Microsoft SqlClient Data Provider for SQL Server. Specifying the type converts the value of the Parameter to the Microsoft SqlClient Data Provider for SQL Server 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 Microsoft SqlClient Data Provider for SQL Server 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 type DbType SqlDbType
Boolean Boolean Bit
Byte Byte TinyInt
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.
Char Inferring a SqlDbType from char isn't supported.
DateTime DateTime DateTime
DateTimeOffset DateTimeOffset DateTimeOffset in SQL Server 2008. Inferring a SqlDbType from DateTimeOffset isn't supported in versions of SQL Server earlier than SQL Server 2008.
Decimal Decimal Decimal
Double Double Float
Single Single Real
Guid Guid UniqueIdentifier
Int16 Int16 SmallInt
Int32 Int32 Int
Int64 Int64 BigInt
Object Object Variant
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.
TimeSpan Time Time in SQL Server 2008. Inferring a SqlDbType from TimeSpan isn't supported in versions of SQL Server earlier than SQL Server 2008.
UInt16 UInt16 Inferring a SqlDbType from UInt16 isn't supported.
UInt32 UInt32 Inferring a SqlDbType from UInt32 isn't supported.
UInt64 UInt64 Inferring a SqlDbType from UInt64 isn't supported.
AnsiString VarChar
AnsiStringFixedLength Char
Currency Money
Date Date in SQL Server 2008. Inferring a SqlDbType from Date isn't supported in versions of SQL Server earlier than SQL Server 2008.
SByte Inferring a SqlDbType from SByte isn't supported.
StringFixedLength NChar
Time Time in SQL Server 2008. Inferring a SqlDbType from Time isn't supported in versions of SQL Server earlier than SQL Server 2008.
VarNumeric Inferring a SqlDbType from VarNumeric isn't supported.
user-defined type (an object with SqlUserDefinedAggregateAttribute SqlClient always returns an Object SqlDbType.Udt if SqlUserDefinedTypeAttribute is present, otherwise Variant

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 isn't representable in the destination type, an OverflowException is thrown.

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.

Derive parameter information

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

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.

Using parameters with a SqlCommand and a stored procedure

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 extra security. Although a stored procedure can be called by passing the stored procedure name followed by parameter arguments as a 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

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 sp_executesql (Transact-SQL).

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 Microsoft SqlClient Data Provider for SQL Server doesn't support the question mark (?) placeholder for passing parameters to a 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)

Example

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.

Note

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();
        using (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();
        }
    }
}

See also