就目前而言，这似乎已经奏效，但我还不打算把它标记为答案，希望有一个更容易的方法。

class ReportKey
{
    private int? IntKey { get; }
    private string StringKey { get; } = string.Empty;
   
    internal ReportKey(int? intKey, string stringKey)
    {
        IntKey = intKey;
        StringKey = stringKey;
    }
    
    public override bool Equals(object obj)
    {
        var item = obj as ReportKey;
    
        if (item == null) return false;
    
        return this.IntKey == item.Intkey &&
           StringKey == item.StringKey;
    }
    
    public override int GetHashCode()
    {
        return $"{IntKey}{StringKey}".GetHashCode();
    }
}

...

TableA.GroupJoin(TableB,
    a => new ReportKey(a.IntKeyA, a.StringKeyA),
    b => new ReportKey(b.NullIntKeyB, b.StringKeyB),
   (tabA, tabB) => new {tabA, tabB});