MVC Model require true

I would create a validator for both Server AND Client side. Using MVC and unobtrusive form validation, this can be achieved simply by doing the following:

Firstly, create a class in your project to perform the server side validation like so:

public class EnforceTrueAttribute : ValidationAttribute, IClientValidatable
{
    public override bool IsValid(object value)
    {
        if (value == null) return false;
        if (value.GetType() != typeof(bool)) throw new InvalidOperationException("can only be used on boolean properties.");
        return (bool)value == true;
    }

    public override string FormatErrorMessage(string name)
    {
        return "The " + name + " field must be checked in order to continue.";
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        yield return new ModelClientValidationRule
        {
            ErrorMessage = String.IsNullOrEmpty(ErrorMessage) ? FormatErrorMessage(metadata.DisplayName) : ErrorMessage,
            ValidationType = "enforcetrue"
        };
    }
}

Following this, annotate the appropriate property in your model:

[EnforceTrue(ErrorMessage=@"Error Message")]
public bool ThisMustBeTrue{ get; set; }

And Finally, enable client side validation by adding the following script to your View:

<script type="text/javascript">
    jQuery.validator.addMethod("enforcetrue", function (value, element, param) {
        return element.checked;
    });
    jQuery.validator.unobtrusive.adapters.addBool("enforcetrue");
</script>

Note: We already created a method GetClientValidationRules which pushes our annotation to the view from our model.

If using resource files to supply the error message for internationalization, remove the FormatErrorMessage call (or just call the base) and tweak the GetClientValidationRules method like so:

public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
{
    string errorMessage = String.Empty;
    if(String.IsNullOrWhiteSpace(ErrorMessage))
    {
        // Check if they supplied an error message resource
        if(ErrorMessageResourceType != null && !String.IsNullOrWhiteSpace(ErrorMessageResourceName))
        {
            var resMan = new ResourceManager(ErrorMessageResourceType.FullName, ErrorMessageResourceType.Assembly);
            errorMessage = resMan.GetString(ErrorMessageResourceName);
        }
    }
    else
    {
        errorMessage = ErrorMessage;
    }

    yield return new ModelClientValidationRule
    {
        ErrorMessage = errorMessage,
        ValidationType = "enforcetrue"
    };
}

I know this is an older post but wanted to share a simple server side way to do this. You create a public property set to true and compare your bool to that property. If your bool is not checked (by default false) the form will not validate.

public bool isTrue
{ get { return true; } }

[Required]
[Display(Name = "I agree to the terms and conditions")]
[Compare("isTrue", ErrorMessage = "Please agree to Terms and Conditions")]
public bool AgreeTerms { get; set; }

Razor code

@Html.CheckBoxFor(m => Model.AgreeTerms, new { id = "AgreeTerms", @checked = "checked" })
<label asp-for="AgreeTerms" class="control-label"></label>
<a target="_blank" href="/Terms">Read</a>
<br />
@Html.ValidationMessageFor(model => model.AgreeTerms, "", new { @class = "text-danger" })
@Html.HiddenFor(x => Model.isTrue)

You could create your own validator:

public class IsTrueAttribute : ValidationAttribute
{
    #region Overrides of ValidationAttribute

    /// <summary>
    /// Determines whether the specified value of the object is valid. 
    /// </summary>
    /// <returns>
    /// true if the specified value is valid; otherwise, false. 
    /// </returns>
    /// <param name="value">The value of the specified validation object on which the <see cref="T:System.ComponentModel.DataAnnotations.ValidationAttribute"/> is declared.
    ///                 </param>
    public override bool IsValid(object value)
    {
        if (value == null) return false;
        if (value.GetType() != typeof(bool)) throw new InvalidOperationException("can only be used on boolean properties.");

        return (bool) value;
    }

    #endregion
}

I tried several solutions but none of them worked completely for me to get both client and server side validation. So what I did in my MVC 5 application to get it to work:

In your ViewModel (for server side validation):

public bool IsTrue => true;

[Required]
[Display(Name = "I agree to the terms and conditions")]
[Compare(nameof(IsTrue), ErrorMessage = "Please agree to Terms and Conditions")]
public bool HasAcceptedTermsAndConditions { get; set; }

In your Razor page (for client side validation):

<div class="form-group">
   @Html.CheckBoxFor(m => m.HasAcceptedTermsAndConditions)
   @Html.LabelFor(m => m.HasAcceptedTermsAndConditions)
   @Html.ValidationMessageFor(m => m.HasAcceptedTermsAndConditions)

   @Html.Hidden(nameof(Model.IsTrue), "true")
</div>

I would just like to direct people to the following Fiddle: https://dotnetfiddle.net/JbPh0X

The user added [Range(typeof(bool), "true", "true", ErrorMessage = "You gotta tick the box!")] to their boolean property which causes server side validation to work.

In order to also have the client side validation working, they added the following script:

// extend jquery range validator to work for required checkboxes
var defaultRangeValidator = $.validator.methods.range;
$.validator.methods.range = function(value, element, param) {
    if(element.type === 'checkbox') {
        // if it's a checkbox return true if it is checked
        return element.checked;
    } else {
        // otherwise run the default validation function
        return defaultRangeValidator.call(this, value, element, param);
    }
}