如何将条件必需属性放入类属性以使用WEB API？

你可以实现自己的ValidationAttribute.也许是这样的:

public class RequireWhenCategoryAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var employee = (EmployeeModel) validationContext.ObjectInstance;
        if (employee.CategoryId == 1)
        {
            return ValidationResult.Success;
        }
        var emailStr = value as String;
        return string.IsNullOrEmpty(emailStr) ? new ValidationResult("Value is required.") : ValidationResult.Success;
    }
}

public sealed class EmployeeModel
{
    [Required]
    public int CategoryId { get; set; }
    [RequireWhenCategory]
    public string Email { get; set; } // If CategoryId == 1 then it is required
}

这只是一个样本.它可能有铸造问题,我不确定这是解决这个问题的最佳方法.