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Parameter Class

Updated: July 2009

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)

[AspNetHostingPermissionAttribute(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
[AspNetHostingPermissionAttribute(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal)]
public class Parameter : ICloneable, IStateManager

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.

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;
    }
  }
}

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

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

Date

History

Reason

July 2009

Enhanced the summary of classes that derive from Parameter.

Customer feedback.

April 2009

Improved the introduction to the code example.

Customer feedback.

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