Avoid NVARCHAR when using ToList() in Linq - sql

Question

I have a property in a class with Column(TypeName) set to VARCHAR but when the linq uses ToList(), the SQL generated by linq converts to NVARCHAR instead. Is there a way to avoid nvarchar conversion which happens when ToList() method is called ?
var codes = xyz.Where(x => x.IsValid).Select(x => x.Code.ToLower()).ToList();
requests = requests.Where(p => codes.Contains(p.Type.ToLower()));
Property(c => c.Type).HasColumnType("varchar").HasMaxLength(3).IsFixedLength();
As shown above, though the Type property has column type set to VARCHAR but linq uses NVARCHAR when ToList() is used.

Answer 1

You can build your own predicate:
public static class PredicateBuilder
{
private static readonly MethodInfo asNonUnicodeMethodInfo =
typeof(EntityFunctions).GetMethod("AsNonUnicode");
private static readonly MethodInfo stringEqualityMethodInfo =
typeof(string).GetMethod("op_Equality");
public static Expression<Func<TEntity, bool>> ContainsNonUnicodeString<TEntity>(
IEnumerable<string> source,
Expression<Func<TEntity, string>> expression)
{
if (source == null) throw new ArgumentNullException("source");
if (expression == null) throw new ArgumentNullException("expression");
Expression predicate = null;
foreach (string value in source)
{
var fragment = Expression.Equal(
expression.Body,
Expression.Call(null,
asNonUnicodeMethodInfo,
Expression.Constant(value, typeof(string))),
false,
stringEqualityMethodInfo);
if (predicate == null)
{
predicate = fragment;
}
else
{
predicate = Expression.OrElse(predicate, fragment);
}
}
return Expression.Lambda<Func<TEntity, bool>>(predicate,
((LambdaExpression)expression).Parameters);
}
}
See more here: https://learn.microsoft.com/da-dk/archive/blogs/diego/workaround-for-performance-with-enumerable-contains-and-non-unicode-columns-against-ef-in-net-4-0