Data is Null. This method or property cannot be called on Null values

You shouldn't be trying to convert the null values from the proc into ints - so before you create the MovieGenre instance you need to check the nullable fields using the SqlDataReader.IsDBNull method:

http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqldatareader.isdbnull.aspx

Assuming that the GenreID and MovieGenreID are nullable ints you could do something like:

movieGenre.Add(new MovieGenre {  MovieID = reader.GetInt32(movieIDIndex),  MovieGenreID = reader.IsDBNull(movieGenreIDIndex) ? null : reader.GetInt32(movieGenreIDIndex),  GenreID = reader.IsDBNull(genreIDIndex) ? null : reader.GetInt32(genreIDIndex)});

This error happens immediately after I enabled C# 8 nullable feature in my Entity Framework Core 3.1 project.

The solution is to change your entity properties to their nullable counterparts. For example,

Change from:

public class Person {  public int Id { get; set; }  public string Name { get;set; }  public string Address { get;set; }}

To:

public class Person {  public int Id { get; set; }  public string Name { get;set; }  public string? Address { get;set; }  //change address to nullable string since it is nullable in database}

Edit your select statement as follows to handle null issue.

SELECT ISNULL(m.MovieID,0) AS MovieID,        ISNULL(g.GenreID,0) AS GenreID,        ISNULL(mg.MovieGenreID,0) AS MovieGenreID,       ISNULL(g.Genre,'') AS GenreFROM --rest of your query...