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Parameter Class

Provides a mechanism that data source controls use to bind to application variables, user identities and choices, and other data. Serves as the base class for all ASP.NET parameter types.

Namespace:  System.Web.UI.WebControls
Assembly:  System.Web (in System.Web.dll)

public class Parameter : ICloneable, IStateManager

The Parameter type exposes the following members.

  NameDescription
Public methodParameter()Initializes a new default instance of the Parameter class.
Protected methodParameter(Parameter)Initializes a new instance of the Parameter class with the values of the original, specified instance.
Public methodParameter(String)Initializes a new instance of the Parameter class, using the specified name.
Public methodParameter(String, DbType)Initializes a new instance of the Parameter class, using the specified name and database type.
Public methodParameter(String, TypeCode)Initializes a new instance of the Parameter class, using the specified name and type.
Public methodParameter(String, DbType, String)Initializes a new instance of the Parameter class, using the specified name, the specified database type, and the specified value for its DefaultValue property.
Public methodParameter(String, TypeCode, String)Initializes a new instance of the Parameter class, using the specified name, the specified type, and the specified string for its DefaultValue property.
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  NameDescription
Public propertyConvertEmptyStringToNullGets or sets a value indicating whether the value that the Parameter object is bound to should be converted to null if it is String.Empty.
Public propertyDbTypeGets or sets the database type of the parameter.
Public propertyDefaultValueSpecifies a default value for the parameter, should the value that the parameter is bound to be uninitialized when the Evaluate method is called.
Public propertyDirectionIndicates whether the Parameter object is used to bind a value to a control, or the control can be used to change the value.
Protected propertyIsTrackingViewStateGets a value indicating whether the Parameter object is saving changes to its view state.
Public propertyNameGets or sets the name of the parameter.
Public propertySizeGets or sets the size of the parameter.
Public propertyTypeGets or sets the type of the parameter.
Protected propertyViewStateGets a dictionary of state information that allows you to save and restore the view state of a Parameter object across multiple requests for the same page.
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  NameDescription
Protected methodCloneReturns a duplicate of the current Parameter instance.
Public methodStatic memberConvertDbTypeToTypeCodeConverts a DbType value to an equivalent TypeCode value.
Public methodStatic memberConvertTypeCodeToDbTypeConverts a TypeCode value to an equivalent DbType value.
Public methodEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodEvaluateUpdates and returns the value of the Parameter object.
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetDatabaseTypeGets the DbType value that is equivalent to the CLR type of the current Parameter instance.
Public methodGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodLoadViewStateRestores the data source view's previously saved view state.
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Protected methodOnParameterChangedCalls the OnParametersChanged method of the ParameterCollection collection that contains the Parameter object.
Protected methodSaveViewStateSaves the changes to the Parameter object's view state since the time the page was posted back to the server.
Protected methodSetDirtyMarks the Parameter object so its state will be recorded in view state.
Public methodToStringConverts the value of this instance to its equivalent string representation. (Overrides Object.ToString().)
Protected methodTrackViewStateCauses the Parameter object to track changes to its view state so they can be stored in the control's ViewState object and persisted across requests for the same page.
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  NameDescription
Explicit interface implemetationPrivate methodICloneable.CloneReturns a duplicate of the current Parameter instance.
Explicit interface implemetationPrivate propertyIStateManager.IsTrackingViewStateInfrastructure. Gets a value indicating whether the Parameter object is saving changes to its view state.
Explicit interface implemetationPrivate methodIStateManager.LoadViewStateInfrastructure. Restores the data source view's previously saved view state.
Explicit interface implemetationPrivate methodIStateManager.SaveViewStateInfrastructure. Saves the changes to the Parameter object's view state since the time the page was posted back to the server.
Explicit interface implemetationPrivate methodIStateManager.TrackViewStateInfrastructure. Causes the Parameter object to track changes to its view state so they can be stored in the control's ViewState object and persisted across requests for the same page.
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The Parameter class represents a parameter in a parameterized SQL query, a filtering expression, or a business object method call that an ASP.NET data source control uses to select, filter, or modify data. Parameter objects are contained in a ParameterCollection object. Parameter objects are evaluated at run time, to bind the values of the variables they represent to whatever method is used by a data source control to interact with data.

Use classes that derive from Parameter with data source and data-bound controls to build Web-based data applications. These parameter classes are used by data source controls to bind specific kinds of values found in Web applications to placeholders in SQL query strings, business object method parameters, and more. The following table lists parameter types that are included in ASP.NET.

ControlParameter

Binds any public property of a Web server control.

FormParameter

Binds a form field.

SessionParameter

Binds a session-state field.

RouteParameter

Binds a route URL parameter.

CookieParameter

Binds a cookie field.

QueryStringParameter

Binds a query-string parameter.

ProfileParameter

Binds a profile field.

Extend the base Parameter class when you want to implement your own custom parameter types.

Parameter objects are very simple: they have a Name and a Type property, can be represented declaratively, and can track state across multiple HTTP requests. All parameters support a DefaultValue property, for cases when a parameter is bound to a value, but the value evaluates to null at run time.

When using a collection of Parameter objects with a data source control, their order in the collection might matter. For more information on how parameters are used, see Using Parameters with the SqlDataSource Control and Using Parameters with the ObjectDataSource Control.

The following example shows how to use the selected value of a DropDownList control in the Where clause of a SQL query. The example uses the ControlParameter class, which derives from the ControlParameter class.

The SelectCommand element defines the query with a parameter named "@Title" where the value from DropDownList1 should go. The ControlParameter element specifies that the "@Title" placeholder will be replaced by the value of the SelectedValue property of the DropDownList1 control. The ControlParameter element is added to the SelectParameters collection of the SqlDataSource control.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
  <head runat="server">
    <title>ASP.NET Example</title>
</head>
<body>
    <form id="form1" runat="server">

      <p><asp:dropdownlist
          id="DropDownList1"
          runat="server"
          autopostback="True">
          <asp:listitem selected="True">Sales Representative</asp:listitem>
          <asp:listitem>Sales Manager</asp:listitem>
          <asp:listitem>Vice President, Sales</asp:listitem>
      </asp:dropdownlist></p>

      <asp:sqldatasource
          id="SqlDataSource1"
          runat="server"
          connectionstring="<%$ ConnectionStrings:MyNorthwind%>"
          selectcommand="SELECT LastName FROM Employees WHERE Title = @Title">
          <selectparameters>
              <asp:controlparameter name="Title" controlid="DropDownList1" propertyname="SelectedValue"/>
          </selectparameters>
      </asp:sqldatasource>

      <p><asp:listbox
          id="ListBox1"
          runat="server"
          datasourceid="SqlDataSource1"
          datatextfield="LastName">
      </asp:listbox></p>

    </form>
  </body>
</html>

The following example is like the previous one, but uses code instead of markup. When the page loads the first time, the DropDownList control has no selected value, and the DefaultValue property of the Parameter object is used.

<%@ Page Language="C#" CodeFile="param1acs.aspx.cs" Inherits="param1acs_aspx" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>ASP.NET Example</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:DropDownList
          runat="server"
          AutoPostBack="True"
          id="DropDownList1">
            <asp:ListItem Value="USA">USA</asp:ListItem>
            <asp:ListItem Value="UK">UK</asp:ListItem>
         </asp:DropDownList>

        <asp:DataGrid
          runat="server"
          id="DataGrid1" />    
    </div>
    </form>
</body>
</html>

The following code shows the code-behind class for the page in the previous example.

public partial class param1acs_aspx : System.Web.UI.Page 
{
    private void Page_Load(object sender, System.EventArgs e)
    {
        SqlDataSource sqlSource = new SqlDataSource(
          ConfigurationManager.ConnectionStrings["MyNorthwind"].ConnectionString,
          "SELECT FirstName, LastName FROM Employees WHERE Country = @country;");

        ControlParameter country = new ControlParameter();
        country.Name = "country";
        country.Type = TypeCode.String;
        country.ControlID = "DropDownList1";
        country.PropertyName = "SelectedValue";

        // If the DefaultValue is not set, the DataGrid does not 
        // display anything on the first page load. This is because 
        // on the first page load, the DropDownList has no 
        // selected item, and the ControlParameter evaluates to 
        // String.Empty.
        country.DefaultValue = "USA";

        sqlSource.SelectParameters.Add(country);

        // Add the SqlDataSource to the page controls collection.
        Page.Controls.Add(sqlSource);

        DataGrid1.DataSource = sqlSource;
        DataGrid1.DataBind();
    }
}

The following code example demonstrates how to extend the Parameter class to create a new parameter type that can be used by data source controls and other controls in data-binding scenarios. A data source control can use a StaticParameter parameter to bind to the value of any object, typically a string, declared on a Web Forms page.

namespace Samples.AspNet {

  using System;
  using System.ComponentModel;
  using System.Security.Permissions;
  using System.Web;
  using System.Web.UI;
  using System.Web.UI.WebControls;

  [AspNetHostingPermission(SecurityAction.Demand, Level=AspNetHostingPermissionLevel.Minimal)]
  public class StaticParameter : Parameter {

    public StaticParameter() {
    }
    // The StaticParameter(string, object) constructor 
    // initializes the DataValue property and calls the 
    // Parameter(string) constructor to initialize the Name property. 
    public StaticParameter(string name, object value) : base(name) {
      DataValue = value;
    }
    // The StaticParameter(string, TypeCode, object) constructor 
    // initializes the DataValue property and calls the 
    // Parameter(string, TypeCode) constructor to initialize the Name and 
    // Type properties. 
    public StaticParameter(string name, TypeCode type, object value) : base(name, type) {
      DataValue = value;
    }
    // The StaticParameter copy constructor is provided to ensure that 
    // the state contained in the DataValue property is copied to new 
    // instances of the class. 
    protected StaticParameter(StaticParameter original) : base(original) {
      DataValue = original.DataValue;
    }

    // The Clone method is overridden to call the 
    // StaticParameter copy constructor, so that the data in 
    // the DataValue property is correctly transferred to the 
    // new instance of the StaticParameter. 
    protected override Parameter Clone() {
      return new StaticParameter(this);
    }
    // The DataValue can be any arbitrary object and is stored in ViewState. 
    public object DataValue {
      get {
        return ViewState["Value"];
      }
      set {
        ViewState["Value"] = value;
      }
    }
    // The Value property is a type safe convenience property 
    // used when the StaticParameter represents string data. 
    // It gets the string value of the DataValue property, and 
    // sets the DataValue property directly. 
    public string Value {
      get {
        object o = DataValue;
        if (o == null || !(o is string))
          return String.Empty;
        return (string)o;
      }
      set {
        DataValue = value;
        OnParameterChanged();
      }
    }

    // The Evaluate method is overridden to return the 
    // DataValue property instead of the DefaultValue. 
    protected override object Evaluate(HttpContext context, Control control) {

      if (context.Request == null)
          return null;

      return DataValue;
    }
  }
}

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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