IObjectSpace.FirstOrDefault<ObjectType>(Expression<Func<ObjectType, Boolean>>) Method

Searches for the first object that matches the specified lambda expression. The generic parameter determines the object’s type.

Namespace: DevExpress.ExpressApp

Assembly: DevExpress.ExpressApp.v23.2.dll

NuGet Package: DevExpress.ExpressApp

Declaration

C#

ObjectType FirstOrDefault<ObjectType>(
    Expression<Func<ObjectType, bool>> criteriaExpression
)
    where ObjectType : class

VB.NET

Function FirstOrDefault(Of ObjectType As Class)(
    criteriaExpression As Expression(Of Func(Of ObjectType, Boolean))
) As ObjectType

Parameters

Name	Type	Description
criteriaExpression	Expression<Func<ObjectType, Boolean>>	A lambda expression to search for an object.

Type Parameters

Name	Description
ObjectType	The Type of an object to be returned.

Returns

Type	Description
ObjectType	The first object that matches the specified lambda expression.

Remarks

The following example uses a Parametrized Action to search for a Person by LastName. After that, this example invokes the default mail client and creates a new draft of a message for that person.

C#

using DevExpress.ExpressApp;
using DevExpress.ExpressApp.Actions;
using DevExpress.Persistent.Base;
using System.Diagnostics;
// ...
public class SendEmailController : ObjectViewController<ListView, Contact> {
    public SendEmailController() {
        ParametrizedAction sendEmailAction = new ParametrizedAction(
            this, "SendEmail", PredefinedCategory.Edit, typeof(string));
        sendEmailAction.Execute += SendEmailAction_Execute;
    }
    private void SendEmailAction_Execute(object sender, ParametrizedActionExecuteEventArgs e) {
        IObjectSpace objectSpace = View.ObjectSpace;
        string contactParamValue = e.ParameterCurrentValue as string;
        if (!string.IsNullOrEmpty(contactParamValue)) {
            Contact contact = objectSpace.FirstOrDefault<Contact>(p => p.LastName == contactParamValue);
            if (!string.IsNullOrEmpty(contact?.Email)) {
                Process.Start($"mailto:{contact.Email}");
            }
        }
    }
}

VB.NET

Imports DevExpress.ExpressApp
Imports DevExpress.ExpressApp.Actions
Imports DevExpress.Persistent.Base
Imports System.Diagnostics
' ...
Public Class SendEmailController
    Inherits ObjectViewController(Of ListView, Contact)
    Public Sub New()
        Dim sendEmailAction As New ParametrizedAction(Me, "SendEmail", 
        PredefinedCategory.Edit, GetType(String))
        AddHandler sendEmailAction.Execute, AddressOf SendEmailAction_Execute
    End Sub
    Private Sub SendEmailAction_Execute(ByVal sender As Object, ByVal e As ParametrizedActionExecuteEventArgs)
        Dim objectSpace As IObjectSpace = View.ObjectSpace
        Dim contactParamValue As String = TryCast(e.ParameterCurrentValue, String)
        If Not String.IsNullOrEmpty(contactParamValue) Then
            Dim contact As Contact = objectSpace.FirstOrDefault(Of Contact) _
            (Function(p) p.LastName = contactParamValue)
            If Not String.IsNullOrEmpty(contact?.Email) Then
                Process.Start($"mailto:{contact.Email}")
            End If
        End If
    End Sub
End Class

Do not implement this method when you implement the IObjectSpace interface in a BaseObjectSpace descendant. The BaseObjectSpace.FirstOrDefault<ObjectType>(Expression<Func<ObjectType, Boolean>>) method invokes a public virtual BaseObjectSpace.FirstOrDefault<ObjectType>(Expression<Func<ObjectType, Boolean>>, Boolean) method. Override the public virtual BaseObjectSpace.FirstOrDefault method to implement an object search.