首先，我们需要创建一个继承自ValidationAttribute类的RequiredIf特性类。在该类中，我们需要实现IsValid方法来进行验证逻辑。在IsValid方法中，我们可以获取到当前字段的值以及其他相关字段的值，并根据条件判断是否满足验证要求。

public class RequiredIfAttribute : ValidationAttribute
{
    private readonly string _otherPropertyName;
    private readonly object _otherPropertyValue;

    public RequiredIfAttribute(string otherPropertyName, object otherPropertyValue)
    {
        _otherPropertyName = otherPropertyName;
        _otherPropertyValue = otherPropertyValue;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var otherPropertyInfo = validationContext.ObjectType.GetProperty(_otherPropertyName);
        var otherPropertyValue = otherPropertyInfo.GetValue(validationContext.ObjectInstance);
        
        if (otherPropertyValue != null && otherPropertyValue.Equals(_otherPropertyValue))
        {
            if (value == null || value.ToString().Trim() == "")
            {
                return new ValidationResult(ErrorMessage);
            }
        }

        return ValidationResult.Success;
    }
}

接下来，我们需要创建一个实现IClientModelValidator接口的RequiredIfValidator类。在该类中，我们需要实现AddValidation方法来注册验证规则。在AddValidation方法中，我们需要使用jQuery的validate插件来添加自定义验证规则和错误消息。

public class RequiredIfValidator : IClientModelValidator
{
    private readonly string _otherPropertyName;
    private readonly object _otherPropertyValue;

    public RequiredIfValidator(string otherPropertyName, object otherPropertyValue)
    {
        _otherPropertyName = otherPropertyName;
        _otherPropertyValue = otherPropertyValue;
    }

    public void AddValidation(ClientModelValidationContext context)
    {
        var errorMessage = GetErrorMessage(context.ModelMetadata);
        var otherPropertyName = _otherPropertyName.Substring(_otherPropertyName.IndexOf(".") + 1);
        var otherPropertyValue = _otherPropertyValue.ToString();

        context.Attributes.Add("data-val", "true");
        context.Attributes.Add("data-val-requiredif", errorMessage);
        context.Attributes.Add("data-val-requiredif-propertyname", otherPropertyName);
        context.Attributes.Add("data-val-requiredif-propertyvalue", otherPropertyValue);
    }

    private string GetErrorMessage(ModelMetadata modelMetadata)
    {
        return ErrorMessageString ?? $"{modelMetadata.DisplayName} is required.";
    }
}

最后，我们需要在Startup.cs文件的ConfigureServices方法中注册该验证规则。我们可以使用AddMvcOptions方法来注册一个实现了IClientModelValidator接口的验证器。

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options =>
    {
        options.ModelMetadataDetailsProviders.Add(new RequiredIfMetadataProvider());
        options.ModelValidatorProviders.Add(new RequiredIfValidatorProvider());
    })
    .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

需要注意的是，我们还需要创建一个RequiredIfMetadataProvider类和RequiredIfValidatorProvider类来提供RequiredIfAttribute和RequiredIfValidator的实例。

public class RequiredIfMetadataProvider : IModelMetadataDetailsProvider
{
    public void CreateBindingMetadata(BindingMetadataProviderContext context)
    {
        if (context.Key.MetadataKind == ModelMetadataKind.Property && context.Key.ModelType == typeof(string))
        {
            var propertyName = context.Key.Name;
            var modelType = context.Key.ContainerType;
            var property = modelType.GetProperty(propertyName);
            var requiredIfAttributes = property.GetCustomAttributes(typeof(RequiredIfAttribute), true);

            if (requiredIfAttributes.Length > 0)
            {
                foreach (RequiredIfAttribute attribute in requiredIfAttributes)
                {
                    context.BindingMetadata.ValidatorMetadata.Add(attribute);
                }
            }
        }
    }

    public void CreateDisplayMetadata(DisplayMetadataProviderContext context)
    {
        // Not needed
    }

    public void CreateValidationMetadata(ValidationMetadataProviderContext context)
    {
        // Not needed
    }
}

public class RequiredIfValidatorProvider : IModelValidatorProvider
{
    public void CreateValidators(ModelValidatorProviderContext context)
    {
        if (context.ModelMetadata is DefaultModelMetadata defaultMetadata)
        {
            var propertyValidators = defaultMetadata.PropertyValidators;

            foreach (var propertyValidator in propertyValidators)
            {
                if (propertyValidator.Attribute.GetType() == typeof(RequiredIfAttribute))
                {
                    context.Results.Add(new ValidatorItem
                    {
                        Validator = new RequiredIfValidator(propertyValidator.Attribute.PropertyName, propertyValidator.Attribute.PropertyValue),
                        IsReusable = true
                    });
                }
            }
        }
    }
}

通过以上步骤，我们就成功地创建了一个名为RequiredIf的自定义验证属性，并将其注册到了ASP.NET Core MVC中。现在，我们可以