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 nameDescription
InputThe parameter is an input parameter. This is the default.
InputOutputThe parameter can perform both input and output.
OutputThe parameter is an output parameter.
ReturnValueThe 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 providerParameter naming syntax
System.Data.SqlClientUses named parameters in the format @parametername.
System.Data.OleDbUses positional parameter markers indicated by a question mark (?).
System.Data.OdbcUses positional parameter markers indicated by a question mark (?).
System.Data.OracleClientUses 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 typeDbTypeSqlDbTypeOleDbTypeOdbcTypeOracleType
byte[]BinaryVarBinary. 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.VarBinaryBinaryRaw
Char``Inferring a SqlDbType from char is not supported.CharCharByte
DateTimeOffsetDateTimeOffsetDateTimeOffset in SQL Server 2008. Inferring a SqlDbType from DateTimeOffset is not supported in versions of SQL Server earlier than SQL Server 2008.DateTime
ObjectObjectVariantVariantInferring an OdbcType from Object is not supported.Blob
StringStringNVarChar. 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.VarWCharNVarCharNVarChar
TimeSpanTimeTime in SQL Server 2008. Inferring a SqlDbType from TimeSpan is not supported in versions of SQL Server earlier than SQL Server 2008.DBTimeTimeDateTime
UInt16UInt16Inferring a SqlDbType from UInt16 is not supported.UnsignedSmallIntIntUInt16
UInt32UInt32Inferring a SqlDbType from UInt32 is not supported.UnsignedIntBigIntUInt32
UInt64UInt64Inferring a SqlDbType from UInt64 is not supported.UnsignedBigIntNumericNumber
``CurrencyMoneyCurrencyInferring an OdbcType from Currency is not supported.Number
``DateDate in SQL Server 2008. Inferring a SqlDbType from Date is not supported in versions of SQL Server earlier than SQL Server 2008.DBDateDateDateTime
``SByteInferring a SqlDbType from SByte is not supported.TinyIntInferring an OdbcType from SByte is not supported.SByte
TimeTime in SQL Server 2008. Inferring a SqlDbType from Time is not supported in versions of SQL Server earlier than SQL Server 2008.DBTimeTimeDateTime
VarNumericInferring a SqlDbType from VarNumeric is not supported.VarNumericInferring an OdbcType from VarNumeric is not supported.Number
user-defined type (an object with SqlUserDefinedAggregateAttributeObject or String, depending the provider (SqlClient always returns an Object, Odbc always returns a String, and the OleDb managed data provider can see eitherSqlDbType.Udt if SqlUserDefinedTypeAttribute is present, otherwise VariantOleDbType.VarWChar (if value is null) otherwise OleDbType.Variant.OdbcType.NVarCharnot supported
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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.

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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.

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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.

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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.

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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.

[!CODE [DataWorks SqlClient.StoredProcedure#1](../CodeSnippet/VS_Snippets_ADO.NET/DataWorks SqlClient.StoredProcedure#1)]

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.

OleDb Example

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;  

Commands and Parameters
DataAdapter Parameters
Data Type Mappings in ADO.NET
ADO.NET Managed Providers and DataSet Developer Center