Pass Expression instead lambda

Question

all! I'll try to write extention for add mapping db column by defaults. I using linq2db

this is my method

    public static void EntityWithDefaults<T>(this FluentMappingBuilder fluentMappingBuilder) {
        fluentMappingBuilder.Entity<T>().HasTableName(typeof(T).Name);
        var item = Expression.Parameter(typeof(T), typeof(T).Name);
        foreach (var prop in typeof(T).GetProperties()) {
            if (prop.Name == "ID")
                fluentMappingBuilder.Entity<T>().Property(x => Expression.Property(item, prop.Name)).IsIdentity().IsPrimaryKey();
            else
                fluentMappingBuilder.Entity<T>().Property(x => Expression.Property(item, prop.Name));
        }
    }

it didn't work... but if I write like this - all is ok

       fluentMappingBuilder.Entity<AppLogLong>()
       .HasTableName("AppLog")
       .Property(x => x.ID).IsPrimaryKey().IsIdentity()
       .Property(x => x.ActionDate)
       .Property(x => x.ActionId)
       .Property(x => x.EmployeeId)
       .Property(x => x.RequestType);

I think my problem is wrong expressions for properties. Could you help me, plz?

Thanks!

What do you mean by it didn't work? Are you getting any error? How are you using the extension method? — Chetan
– Chetan, Commented Nov 25, 2019 at 11:09
There are no errors. After my extension method execution fluentMappingBuilder had no any properties, but ID. — RomanReaL D
– RomanReaL D, Commented Nov 25, 2019 at 11:14

canton7 · Accepted Answer · 2019-11-25 12:11:08Z

2

x => x.ID is not the same as x => Expression.Property(item, "ID").

What you want to do is probably:

foreach (var prop in typeof(T).GetProperties()) {
    var parameter = Expression.Parameter(typeof(T), "x");
    var property = Expression.Property(parameter, prop);
    var cast = Expression.Convert(property, typeof(object));
    var lambda = Expression.Lambda<Func<T, object>>(cast, parameter);
    if (prop.Name == "ID")
        fluentMappingBuilder.Entity<T>().Property(lambda).IsIdentity().IsPrimaryKey();
    else
        fluentMappingBuilder.Entity<T>().Property(lambda);
}

That is, we have to construct the entire LambdaExpression ourselves.

edited Nov 25, 2019 at 12:11

answered Nov 25, 2019 at 11:39

canton7

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4 Comments

Nkosi Over a year ago

This is more or less the answer. This will need to be done in the for loop for each property

RomanReaL D Over a year ago

Thanks, but I have error with cast Int32 to object here var lambda = Expression.Lambda<Func<T, object>>(property, parameter);

RomanReaL D Over a year ago

@canton7 cannot convert from 'System.Linq.Expressions.UnaryExpression' to 'System.Linq.Expressions.ParameterExpression' on this var lambda = Expression.Lambda<Func<T, object>>(property, cast); cast is red underlined

RomanReaL D Over a year ago

@canton7 Thank you, very much! I was stuck on this stupid thing))

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