Unable to cast object of type 'System.Int32' to type 'System.String' in DataReader.GetString()

Your column doesn't have the type string. Apparently it's int. So use:

dr.getInt32(1).ToString()

or even

dr.GetValue(1).ToString()

which should be more roubst to type changes in the database.

As some sort of general advice I try to follow at least:

• Select only what you need. This has mostly performance reasons and the reason that you have to state the column names explicitly, thereby getting at least a sensible error if you change your schema incompatibly.

• Access the fields using their names, e.g.

  dr.GetGuid(dr.GetOrdinal("id"))
  

  Such a thing can also be nicely solved by an extension method:

  public T GetFieldValue<T>(this DbDataReader reader, string columnName)
  {
      return reader.GetFieldValue<T>(reader.GetOrdinal(columnName));
  }
  

Side note: Including stack traces (or at least saying which line in your code the exception comes from) can be helpful to others trying to help you. As you can see from the wild guesses what the culprit could be. My guess would be that the stack trace looks somewhat like this:

SqlDataReader.GetString
YourCode.YourMethod

and that GetString looks more or less like this:

public string GetString(int index)
{
    return (string) GetValue(index);
}

Your column doesn't seem to have type int. To avoid things like this, you can use the columnnames instead of indexes.

try
{
    SqlCeCommand com = new SqlCeCommand("select * from Category_Master", con);
    SqlCeDataReader dr = com.ExecuteReader();
    while(dr.Read()){
        string name = dr["yourColumnName"].ToString();
        cmbProductCategory.Items.Add(name);
    }
}
catch(Exception ex)
{
    System.Windows.Forms.MessageBox.Show(ex.Message, System.Windows.Forms.Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Error);
}

you can try the following conversion:

  Convert.ToInt32(yourstring)

If you are encountering the error "Unable to cast object of type 'System.Int32' to type 'System.String'" after upgrading your project and previously used a value conversion like this:

.HasConversion(
    v => v.ToString(),
    v => int.Parse(v)
);

This might have worked in earlier versions of EF Core but may cause issues in later versions, such as .NET 8.

A simpler and more reliable approach is to use .HasConversion(), which allows EF Core to handle the type conversion more effectively. For example:

builder.Entity<SomeEntity>(entry =>
{
    entry.ToTable("MyTable")
        .HasKey(c => c.MyPrimaryKey);
        
    entry.Property(c => c.MyPrimaryKey)
        .HasMaxLength(255)
        .HasColumnName("ID")
        .HasConversion<int>();
});

This approach resolves the type conversion issue and is more straightforward.

Ok OK. Its solved....

Here is the code..

        try
        {
            SqlCeCommand com = new SqlCeCommand("select CategoryName from Category_Master", con);
            SqlCeDataReader dr = com.ExecuteReader();
            while(dr.Read()){
                string name = dr.GetString(0);
                cmbProductCategory.Items.Add(name);
            }
        }
        catch(Exception ex)
        {
            System.Windows.Forms.MessageBox.Show(ex.Message, System.Windows.Forms.Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

I changed the sqlcommand to a single value and changed column number for dr.getstring() to 0.. it worked. Thank you guys for the help.. I expect more because i am only half way in my project..

Use Column name in your query then specify that column name into the reader.

SqlCeCommand com = new SqlCeCommand("select catg from Category_Master", con);
                SqlCeDataReader dr = com.ExecuteReader();
                while(dr.Read()){
                    string name = dr("catg").ToString();
                    cmbProductCategory.Items.Add(name);
                }