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<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:Validators="clr-namespace:XamarinFormValidation.Validators.Implementations"
             xmlns:Behaviors="clr-namespace:XamarinFormValidation.Behaviors"
             xmlns:local="clr-namespace:XamarinFormValidation" x:Class="XamarinFormValidation.MainPage">
    <StackLayout Padding="15">
        <Label>Enter the mobile number</Label>
        <Entry>
            <Entry.Behaviors>

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...
namespace XamarinFormValidation.Behaviors
{
    public class ValidationBehavior : Behavior<View>
    {
        ...
        public bool Validate()
        {
            ...
        }

ameerthehacker

...
<Entry>
    <Entry.Behaviors>
        <Behaviors:ValidationBehavior Group="{x:Reference form}" PropertyName="Text">
            <Behaviors:ValidationBehavior.Validators>
                <v:FormatValidator Format="^[0-9]{6}$" Message="Village Code must be 6 digit" />
                <v:RequiredValidator />
            </Behaviors:ValidationBehavior.Validators>
        </Behaviors:ValidationBehavior>
    </Entry.Behaviors>

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...
namespace XamarinFormValidation.Behaviors
{
    public class ValidationBehavior : Behavior<View>
    {
        ...
        public bool Validate()
        {
            bool isValid = true;
            string errorMessage = "";

ameerthehacker

using Xamarin.Forms;
using System.Collections.ObjectModel;
using XamarinFormValidation.Validators.Contracts;
using XamarinFormValidation.Validators.Implementations;
using System.ComponentModel;

namespace XamarinFormValidation.Behaviors
{
    public class ValidationBehavior : Behavior<View>
    {

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using Xamarin.Forms;
using XamarinFormValidation.Validators.Contracts;

namespace XamarinFormValidation.Validators.Implementations
{
    public class BasicErrorStyle: IErrorStyle
    {
        public void ShowError(View view, string message)
        {
            StackLayout layout = view.Parent as StackLayout;

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using Xamarin.Forms;

namespace XamarinFormValidation.Validators.Contracts
{
    public interface IErrorStyle
    {
        void ShowError(View view, string message);
        void RemoveError(View view);
    }
}

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using System.Text.RegularExpressions;
using XamarinFormValidation.Validators.Contracts;

namespace XamarinFormValidation.Validators.Implementations
{
    public class FormatValidator: IValidator
    {
        public string Message { get; set; } = "Invalid format";
        public string Format { get; set; }

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using XamarinFormValidation.Validators.Contracts;

namespace XamarinFormValidation.Validators.Implementations
{
    public class RequiredValidator: IValidator
    {
        public string Message { get; set; } = "This field is required";

        public bool Check(string value)
        {

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namespace XamarinFormValidation.Validators.Contracts
{
    public interface IValidator
    {
        string Message { get; set; }
        bool Check(string value);
    }
}