How do you validate Blazor forms inside the bootstrap modal?

You have to use the EditContext with EditForm to validate the forms inside a bootstrap modal.

@using System.ComponentModel.DataAnnotations;

<div id="modalDialog" class="modal" tabindex="-1" role="dialog">
    <div class="modal-dialog" role="document">
        <div class="modal-content">
            <div class="modal-body">
                <EditForm EditContext="@EC">
                    <DataAnnotationsValidator />
                    <ValidationSummary />

                    <div class="form-group d-flex justify-content-between">
                        <label class="col-form-label col-3" for="name">Name</label>
                        <InputText bind-Value="@model.Name" id="name" Class="form-control" />
                    </div>
                    @if (EC.Validate())
                    {
                        <button type="button" onclick="@SubmitHandler" class="btn btn-primary" data-dismiss="modal">Submit</button>
                    }
                    else
                    {
                        <button type="button" onclick="@SubmitHandler" class="btn btn-primary">Submit</button>
                    }
                </EditForm>
            </div>
        </div>
    </div>
</div>
<button data-toggle="modal" data-target="#modalDialog" class="btn btn-primary">Open</button>

@code {
    public class ModelClass
    {
        [Required]
        public string Name { get; set; }
    }
    private ModelClass model { get; set; } = new ModelClass();
    private EditContext EC { get; set; }
    private void SubmitHandler()
    {
        Console.WriteLine("Submit");
    }
    protected override void OnInitialized()
    {
        EC = new EditContext(model);
        base.OnInitialized();
    }
    }