Library with common value validators to be used in UI application (e.g. ASP.Net Core, Xamarin, UWP, WPF). The library is based on the ideas from the following book:

• Enterprise Application Patterns using Xamarin.Forms

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Nuget

• IValidationRule<T>: Generic interface for the rules
• IValidatable: Interface to handle validation
• IValidatableValue<T>: Generic interface for a validatable value
• IValidatableValue: Non geniric interface for validatable value (mostly implemented explizit)

• ContainsValidationRule - Check, if the value is in the given collection (with possibility to inverse the result)
• EmailValidationrule - Check, if the value is a valid email address
• EqualValuesValidationRule<T> - Compare two values for equality
• NullValidationRule - Check, if the value is null
• PhoneNumberValidationRule - Check, if the value is valid phone number (with allowed separators -, (), .)
• RangeValidationRule<T> / PrimitiveRangeValidationRule - Compare the value to a given min and max values
• RegexValidationRule - Check, if the value match the given regular expression
• RequiredValidationRule - Check, if the string value is set (with a flag, for white spaces treaded as valid values)
• StringLengthValidationRule - Check the max or min length of a string
• GreaterThanValidationRule - Check, if the value is greater (or equal) to a given value
• SmallerThanValidationRule - Check, if the value is smaller (or equal) to a given value
• RevalidateOtherValueValidationRule - Trigger the validation of another validatable value

Initialize your validatable values in your view model.

MyValidatableValue = new ValidatableValue<string>
{
    ValidationRules = new IValidationRule<string>[]
    {
        new ContainsValidationRule<string>(
            errorMessage: "Value not allowed",
            checkCollection: new[] {"test", "example", "fake"},
            nullOrEmptyIsValid: true),
        new StringLengthValidationRule(
            errorMessage: "The value should have at least 2 characters
            length: 2,
            checkMaxLength: false),
        new StringLengthValidationRule(
            errorMessage: "The value should have at maximum 16 charachters",
            length: 16),
        new RequiredValidationRule(errorMessage: "Value is required")
    }
};

Bind the value in your XAML code.

...
<Label Text="My value:" />
<Entry Text="{Binding MyValidatableValue.Value}"
       BackgroundColor="{Binding MyValidatableValue.IsValid, Converter={StaticResource ValidationColorConverter}}"/>
<Label Text="{Binding MyValidatableValue.FirstError}"
       Style="{StaticResource ErrorMessage}" />
...