Entity Framework calling MAX on null on Records

entity-framework

Yes, casting to Nullable of T is the recommended way to deal with the problem in LINQ to Entities queries. Having a MaxOrDefault() method that has the right signature sounds like an interesting idea, but you would simply need an additional version for each method that presents this issue, which wouldn't scale very well.

This is one of many mismatches between how things work in the CLR and how they actually work on a database server. The Max() method’s signature has been defined this way because the result type is expected to be exactly the same as the input type on the CLR. But on a database server the result can be null. For that reason, you need to cast the input (although depending on how you write your query it might be enough to cast the output) to a Nullable of T.

Here is a solution that looks slightly simpler than what you have above:

var version = ctx.Entries 
    .Where(e => e.Competition.CompetitionId == storeCompetition.CompetitionId) 
    .Max(e =>(int?)e.Version);

Hope this helps.

Try this to create a default for your max.

int version = ctx.Entries 
    .Where(e => e.Competition.CompetitionId == storeCompetition.CompetitionId) 
    .Max(e =>(int?)e.Version) ?? 0;

You could write a simple extension method like this, it returns the default value of type T if no records exist and is then apply Max to that or the query if records exist.

public static T MaxOrEmpty<T>(this IQueryable<T> query)
{
    return query.DefaultIfEmpty().Max();
}

and you could use it like this

maxId = context.Competition.Select(x=>x.CompetitionId).MaxOrEmpty();

I couldnt take no for an answer :) I have tested the below and it works, I havent checked the SQL generated yet so be careful, I will update this once I have tested more.

var test = ctx.Entries
    .Where(e => e.Competition.CompetitionId == storeCompetition.CompetitionId)
    .MaxOrDefault(x => x.Version);

public static TResult? MaxOrDefault<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)
    where TResult : struct
{
    return source
        .Select(selector)
        .Cast<TResult?>()
        .Max();
}

Entity Framework calling MAX on null on Records

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