Save all string values in lowercase

I would like to lower case all string values before saving them do db.

Is there any way NHibernate can do this and how ? Also are there any performance implications that I should be aware of ?

One way to achieve it would be introducing custom type for conversion. Something like:

[Serializable]
public class LowerCaseStringType : AbstractStringType, ILiteralType
{
    public LowerCaseStringType() : base(new StringSqlType())
    {
        //To avoid NHibernate to issue update on flush when the same string is assigned with different casing
        Comparer = StringComparer.OrdinalIgnoreCase;
    }

    public override string Name { get; } = "LowerCaseString";

    public override void Set(DbCommand cmd, object value, int index, ISessionImplementor session)
    {
        base.Set(cmd, ((string) value)?.ToLowerInvariant(), index, session);
    }

    //Called when NHibernate needs to inline non parameterized string right into SQL. Not sure if you need it
    string ILiteralType.ObjectToSQLString(object value, Dialect.Dialect dialect)
    {
        return "'" + ((string) value).ToLowerInvariant() + "'";
    }

    //If you also want to retrieve all values in lowercase than also override Get method
}

Than you can either map required properties with this type like:

<property name="Name" type="YourNamespace.LowerCaseStringType, YourAssemblyName">

Or even register it as default type for all string mappings (at least it's true for latest NHibernate 5.2):

//Somewhere before SessionFactory is created
TypeFactory.RegisterType(typeof(string), new LowerCaseStringType(), new[] {"string", "String"});