In ASP.NET Core MVC, I have multiple forms on a page, and the validation summaries show on both forms

If I have two forms on a page, and submit one the same validation summary shows for both.

I am using an Umbraco 10 SurfaceController but I don't know that this is relevant.

The controller looks like this:

[HttpPost]
[ValidateUmbracoFormRouteString]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Login(LoginForm model)
{
	if (!ModelState.IsValid)
	{
		ModelState.AddModelError(string.Empty, "Please provide username and password");
		return CurrentUmbracoPage();
	}
	//...snipped...
}

The validation summary looks like this

@Html.ValidationSummary()

I have another similar form for registration and the registration validation shows in the header login area when I submit that form.

I looked into the 'prefix' parameter but I failed to find any documentation regarding it's use.

@Html.ValidationSummary() just generates IHtmlContent accroding to your ModelState error,it doesn't know which form you posted at all

In order to decide where the error message should be rendered,you could add a hidden input pass a value related the form to controller

For Example,I tried with a minimal example as below:

public class Entity1
{
public int Id { get; set; }
[Required]
public string? Name { get; set; }
[Required]
public string? Prop1 { get; set; }
}
public class Entity2
{
public int Id { get; set; }
[Required]
public string? Name { get; set; }
[Required]
public string? Prop2 { get; set; }
}
public class MyViewModel
{
public Entity1? Entity1 { get; set; }
public Entity2? Entity2 { get; set; }
}

View:

<div class="row">
<div class="col-md-4">
<form asp-action="Create">
@if (ViewData["Form"] as string == "form1")
{
@Html.ValidationSummary()
}
<div class="form-group">
<label asp-for="Entity1.Name" class="control-label"></label>
<input asp-for="Entity1.Name" class="form-control" />
<span asp-validation-for="Entity1.Name" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="Entity1.Prop1" class="control-label"></label>
<input asp-for="Entity1.Prop1" class="form-control" />
<span asp-validation-for="Entity1.Prop1" class="text-danger"></span>
</div>
<input name="form" value="form1" hidden />
<div class="form-group">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</form>
<form asp-action="Create" name="">
@if (ViewData["Form"] as string == "form2")
{
@Html.ValidationSummary()
}
<div class="form-group">
<label asp-for="Entity2.Name" class="control-label"></label>
<input asp-for="Entity2.Name" class="form-control" />
<span asp-validation-for="Entity2.Name" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="Entity2.Prop2" class="control-label"></label>
<input asp-for="Entity2.Prop2" class="form-control" />
<span asp-validation-for="Entity2.Prop2" class="text-danger"></span>
</div>
<input name="form" value="form2" hidden />
<div class="form-group">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</form>
</div>
</div>
<div>
<a asp-action="Index">Back to List</a>
</div>

Controller:

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create(MyViewModel myvm)
{
.......
ModelState.AddModelError("", "ModelErr");
var formname = HttpContext.Request.Form["form"].ToString();
ViewData["Form"] = formname;
return View(myvm);
}

The result: