1. 首先，在需要自定义验证的Model中，添加需要验证的两个属性。

public class MyModel { public string Field1 { get; set; }

public string Field2 { get; set; }

}

2. 接着，定义一个新的自定义ValidationAttribute类，命名为StringLengthComparisonAttribute，并继承ValidationAttribute类。

public class StringLengthComparisonAttribute : ValidationAttribute { private readonly string _dependentProperty; private readonly ComparisonOperator _comparisonOperator; private readonly int _dependentPropertyValue;

public StringLengthComparisonAttribute(string dependentProperty, ComparisonOperator comparisonOperator, int dependentPropertyValue)
{
    _dependentProperty = dependentProperty;
    _comparisonOperator = comparisonOperator;
    _dependentPropertyValue = dependentPropertyValue;
}

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    var dependentProperty = validationContext.ObjectType.GetProperty(_dependentProperty);
    if (dependentProperty == null)
    {
        return new ValidationResult($"Unknown property {_dependentProperty}");
    }

    var dependentPropertyValue = dependentProperty.GetValue(validationContext.ObjectInstance, null);

    if (!int.TryParse(dependentPropertyValue.ToString(), out int intDependentPropertyValue))
    {
        return new ValidationResult($"Property {_dependentProperty} is not of type int");
    }

    if (value == null)
    {
        return ValidationResult.Success;
    }

    var valueString = value.ToString();

    var dependentPropertyValueString = dependentPropertyValue.ToString();
    
    if(_comparisonOperator == ComparisonOperator.GreaterThan) 
    {
        if (valueString.Length > intDependentPropertyValue)
        {
            return new ValidationResult($"The length of {validationContext.DisplayName} must be greater than the length of {_dependentProperty}");
        }
    } 
    else if(_comparisonOperator == ComparisonOperator.LessThan) 
    {
        if (valueString.Length < intDependentPropertyValue)
        {
            return new ValidationResult($"The length of {validationContext.DisplayName} must be less than the length of {_dependentProperty}");
        }
    }
    
    return ValidationResult.Success;
}

}

在StringComparisonOperator枚举中定义比较运算符。

public enum ComparisonOperator { LessThan, GreaterThan }

3. 最后，在需要验证的属性