How do I define a C# / ASP.NET MVC String class with a MaxLength that works with Entity Framework Core?

Question

I would like to define a class for different strings which Entity Framework Core will recognise and apply certain rules without data annotations.

For instance I would like to replace the following string property with a string object:

[MaxLength(20)]
public string PhoneNumber { get; set; }

to be replaced with:

public PhoneNumber PhoneNumber { get; set; }

In this scenario, the PhoneNumber class will have a max length of 20 when called by Entity Framework Core.

public class PhoneNumber
{
    private readonly string value;
    private readonly long maxLength;

    public PhoneNumber(string value, long maxLength = long.MaxValue)
    {
        if (value != null && value.Length > maxLength)
        {
            throw new InvalidOperationException(string.Format("Value is limited to {0} characters.", maxLength.ToString()));
        }

        this.value = value;
        this.maxLength = maxLength;
    }

    public override string ToString()
    {
        return value;
    }

    public static implicit operator PhoneNumber(string phoneNumber)
    {
        return new PhoneNumber(phoneNumber);
    }

    public static implicit operator String(PhoneNumber phoneNumber)
    {
        return phoneNumber.value;
    }
}

Out of curiosity, what is the use-case you're solving for with this strategy? Also wondering if a struct would be better than a class here but I think it's always going to get allocated on the heap so maybe it doesn't matter. — Sal
– Sal, Commented Jul 19 at 16:20

David Browne - Microsoft · Accepted Answer · 2025-07-19 17:30:26Z

3

You can use a Value Conversion to convert to/from a string for storage.

https://learn.microsoft.com/en-us/ef/core/modeling/value-conversions?tabs=data-annotations

answered Jul 19 at 17:30

David Browne - Microsoft

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