MaxLength annotation to maxlength input property

asp.net-core, asp.net-core-mvc, c#, validation

Solution

You may need to implement this functionality via a TagHelper which can read this attribute and add it to the element when rendered, as the default `asp-for` one won't handle this.

Extending the Input TagHelper

Try declaring a TagHelper as follows within your project, which will etend the existing `asp-for` helper and handle reading any existing attributes / metadata and appending the necessary attributes to the element:

namespace YourProject.TagHelpers
{
    [HtmlTargetElement("input", Attributes = "asp-for")]
    public class MaxLengthTagHelper : TagHelper
    {
        public override int Order { get; } = 999;

        [HtmlAttributeName("asp-for")]
        public ModelExpression For { get; set; }

        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            base.Process(context, output);

            // Process only if 'maxlength' attr is not present
            if (context.AllAttributes["maxlength"] == null) 
            {
                // Attempt to check for a MaxLength annotation
                var maxLength = GetMaxLength(For.ModelExplorer.Metadata.ValidatorMetadata);
                if (maxLength > 0)
                {
                    output.Attributes.Add("maxlength", maxLength);
                }
            }
        }

        private static int GetMaxLength(IReadOnlyList<object> validatorMetadata)
        {
            for (var i = 0; i < validatorMetadata.Count; i++)
            {
                var stringLengthAttribute = validatorMetadata[i] as StringLengthAttribute;
                if (stringLengthAttribute != null && stringLengthAttribute.MaximumLength > 0)
                {
                    return stringLengthAttribute.MaximumLength;
                }

                var maxLengthAttribute = validatorMetadata[i] as MaxLengthAttribute;
                if (maxLengthAttribute != null && maxLengthAttribute.Length > 0)
                {
                    return maxLengthAttribute.Length;
                }
            }
            return 0;
        }
    }
}

Using The TagHelper

Then add a reference to it, either directly in your specific view, or globally in the `_ViewImports.cshtml` file as seen below:

@using YourProject
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@addTagHelper *, YourProject

Once added, this extended TagHelper should automatically decorate your element with the appropriate `maxlength` attribute, if present on your property:

<!-- Input -->
<label asp-for="PostCode"></label>
<input style="font-weight: normal;" class="form-control" placeholder="Post Code" asp-for="PostCode" data-val="true" autofocus />

<!-- Rendered -->
<label for="PostCode">Post Code</label>
<input style="font-weight: normal;" class="form-control" placeholder="Post Code" data-val="true" autofocus="" type="text" data-val-maxlength="Maximum number of characters that can be entered is 8!" data-val-maxlength-max="8" id="PostCode" name="PostCode" value="" maxlength="8">

Problem

Original source