Ajouter un message d'erreur à @Html.ValidationSummary

Côté client:

function YourCustomValidator() {
    //do your validation logic here via JavaScript
    return true; //or false based on your validation logic
}
$(document).ready(function () {
    //take your own form-selector like ("form", this)
    $("form", this).first().submit(function () {
        return (YourCustomValidator() && $(this).valid());
    });
});

OU côté serveur:

Pense que vous avez un modèle comme celui-ci:

public class Test {
    [Required]
    [StringLength(100)]
    public string FullName { get; set; }
}

et quand vous êtes à la validation:

if(ModelState.IsValid) { //default validations run here
    if(/* some custom validations run here, there is an error about "FullName" */){
        //you should set the "key" for Model-Error to "FullName"
        ModelState.AddModelError("FullName","error-message goes here")
    }
    if(/* some custom validations run here, the error is global, not on "FullName" */){
        //you should set the "key" for Model-Error to an empty-string
        ModelState.AddModelError("","error-message goes here")
    }
    //also you can test for model-errors again like this:
    if(ModelState.IsValid) { //if you add any error above, this will be "false"

    }
}