Select distinct column from Entity Framework with dynamic column name

The only problem I see with Linq.Dynamic is that there were no updates since 2013 and the project is pretty much dead

I would handle it via extensions, and improve reflection performance via caching (not elaborated here)

Extensions:

public static class QueryableExtensions
{
    public static IReadOnlyCollection<TResult> GetDistinctValuesForProperty<T, TResult>(this IQueryable<T> query, Expression<Func<T, TResult>> propertyAccess)
    {
        return SelectDistinct(query, propertyAccess).ToList();
    }

    public static IReadOnlyCollection<object> GetDistinctValuesForProperty<TSource>(this IQueryable<TSource> query, string propertyName)
    {
        var unboundFuncType = typeof(Func<,>);
        var unboundExprType = typeof(Expression<>);

        var sourceType = typeof(TSource); // TSource

        var resultType = typeof(TSource)
            .GetProperty(propertyName)
            .PropertyType; // TResult

        // Func<TSource, TResult>
        var funcType = unboundFuncType.MakeGenericType(new [] { sourceType, resultType });

        // Expression<Func<TSource, TResult>>
        var expressionType = unboundExprType.MakeGenericType(new [] { funcType });

        // Instance of Expression<Func<TSource, TResult>>, for example x => x.Name
        var propertyAccess = typeof(StringExtensions)
            .GetMethod(nameof(StringExtensions.AsPropertyExpression), new[] { typeof(string) })
            .MakeGenericMethod(new [] { sourceType, resultType })
            .Invoke(null, new object[] { propertyName });

        // SelectDistinct query transform
        var selectDistinctMethod = typeof(QueryableExtensions)
            .GetMethod(nameof(QueryableExtensions.SelectDistinct), BindingFlags.NonPublic | BindingFlags.Static)
            .MakeGenericMethod(new [] { sourceType, resultType });

        // IQueryable<TSource> ==> IQueryable<TResult>
        var result = selectDistinctMethod.Invoke(null, new object[] { query, propertyAccess });

        // Cast to object via IEnumerable and convert to list
        return ((IEnumerable)result).Cast<object>().ToList();
    }

    private static IQueryable<TResult> SelectDistinct<TSource, TResult>(this IQueryable<TSource> query, Expression<Func<TSource, TResult>> propertyAccess)
    {
        return query.Select(propertyAccess).Distinct();
    }
}

public static class StringExtensions
{
    public static Expression<Func<T, TResult>> AsPropertyExpression<T, TResult>(this string propertyName)
    {
        var parameter = Expression.Parameter(typeof(T), "x");
        var property = typeof(T).GetProperty(propertyName);
        var body = Expression.MakeMemberAccess(parameter, property);
        return Expression.Lambda<Func<T, TResult>>(body, parameter);
    }
}

Usage:

public class Person
{
    public string Name { get; }
    public int Age { get; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }
}

var people = new Person[]
{
    new Person("John", 25), new Person("Peter", 25), new Person("Sean", 25),
    new Person("John", 32), new Person("Peter", 32),
};

var query = people.AsQueryable();

var namePropertyExpression = "Name".AsPropertyExpression<Person, string>();
var agePropertyExpression = "Age".AsPropertyExpression<Person, int>();

// When you know the result type
var names1 = query.GetDistinctValuesForProperty(x => x.Name);
var ages1 = query.GetDistinctValuesForProperty(x => x.Age);

// When you know the result type, but you may want to reuse the property expression
var names2 = query.GetDistinctValuesForProperty(namePropertyExpression);
var ages2 = query.GetDistinctValuesForProperty(agePropertyExpression);

// When you just know the property name
var names3 = query.GetDistinctValuesForProperty("Name");
var ages3 = query.GetDistinctValuesForProperty("Age");

Finally I found a solution. I need to include reference to System.Linq.Dynamic (downloaded by nuget), and use the "Select" method that accept String to reference column.

using System.Linq.Dynamic;

public static async Task<IEnumerable<Object>> GetDistinctValuesForProperty<T>(this IQueryable<T> query, String PropertyName)
  {
  return await query.Select(PropertyName).Distinct().ToListAsync();
  }

and call as

String ColumnName = "DateTimeInsert";
DbSet<Log> oDbSet = _uow.DbContext.Set<Log>();

Array DistinctValues;
if (typeof(Log).GetProperty(ColumnName) != null)
  {
  DistinctValues = (await oDbSet.GetDistinctValuesForProperty(ColumnName)).ToArray();
  }
else
  {
  DistinctValues = new object[0];
  }

I need to use array vs ienumerable due to a cast problem in case of datetime types

You can create a generic selector method using Expressions

public static Func<T, T> SelectorFunc<T>(string[] columns) {
// input parameter "o"
var xParameter = Expression.Parameter(typeof(T), "o");

// new statement "new Data()"
var xNew = Expression.New(typeof(T));

// create initializers
var bindings = columns.Select(o => o.Trim())
 .Select(o =>
    {

        // property "Field1"
        var mi = typeof(T).GetProperty(o);

        // original value "o.Field1"
        var xOriginal = Expression.Property(xParameter, mi);

        // set value "Field1 = o.Field1"
    return Expression.Bind(mi, xOriginal);
        }
        );

    // initialization "new Data { Field1 = o.Field1, Field2 = o.Field2 }"
        var xInit = Expression.MemberInit(xNew, bindings);

        // expression "o => new Data { Field1 = o.Field1, Field2 = o.Field2 }"
        var lambda = Expression.Lambda<Func<T, T>>(xInit, xParameter);

       // compile to Func<Data, Data>
     return lambda.Compile();
}

using it would be

T.Select( SelectorFunc<T>( new string[]{ "Column" } ) ).Distinct().ToList();

You can also use it any other linq functions like

T.Select( SelectorFunc<T>( new string[]{ "Column" } ) ).Where();
T.Select( SelectorFunc<T>( new string[]{ "Column" } ) ).AsQueryable();

for additional reference you can see the full OP here LINQ : Dynamic select