I’ve been in the need of converting an Excel files with its own defined columns to a new one with dynamic columns, and I was kinda confused on how to do it properly without some serious performance issues.

I came up to a tutorial that you can find here that uses .NET ExpandoObject which pretty much lets you create an object and add dynamic memebers.


The Microsoft definition is:

Represents an object whose members can be dynamically added and removed at run time.

And it has a few remarks:

The ExpandoObject class enables you to add and delete members of its instances at run time and also to set and get values of these members. This class supports dynamic binding, which enables you to use standard syntax like sampleObject.sampleMember instead of more complex syntax like sampleObject.GetAttribute(“sampleMember”).

Current behaviour

We have an ExcelExportService that, by passing a List<T>, in this case ExcelItem, will use Reflection to create an xlsx file.

So far, our code looks like this:

class ExcelItem
    public int Id { get; set; }
    public string Name { get; set; }
    public string Surname { get; set; }
    public string Age { get; set; }
    public string CreatedAt { get; set; }
    public string UpdatedAt { get; set; }

ExcelItem is the object with all the properties that are being used to generate the excel file.

public static byte[] GetExcelBytes() {
    List<ExcelItem> excelItems = new List<ExcelItem>();
    ExcelExportService exportService = new ExcelExportService();

    foreach (var items in itemsToExcel)
        excelItems.Add(new ExcelItem()
            Id = item.Id,
            Name = item.Name,
            Surname = item.Surname,
            Age = item.Age,
            CreatedAt = item.CreatedAt
            UpdatedAt = item.UpdatedAt

    return exportService.ExportToExcel(excelItems);

This approach works perfectly, the xlsx file is generated without problems with each column being each property, you would use this GetExcelBytes method like this, for example, to save it into a file:

File.WriteAllBytes("path-to-somewhere/my-file.xlsx", GetExcelBytes());

The problem

Since what we are doing right know is using all the properties of the ExcelItem object, all I want is to actually not use all of them, just using maybe 2 o 3 of them.

Also a requirement is that I don’t want to be changing the code, eveything should be done by the administrator, who will decide which columns should be shown, and he may or may not know the properties in the code.

TLDR: everything should be dynamic, we have an object with properties and we must make sure that the generated file have N properties of that object, but obviously not hardcoded.

Dynamic columns

The change would be to use a dynamic object, because we would like to set what properties of the object will be used to generate the list of columns.

Let’s say that we have an object with a lot of properties, like, a ton of them, and we don’t really want to be changing the ExcelItem object everytime that we make a change, we create a ColumnExcelItem table, which will be used to generate that ExcelItem.

Database structure

We save this definition in our database with something like this:

Id    PropertyName
1     Name
2     Surname
3     Age

Note that we have less values that the existing properties of the ExcelItem object, with it having 6 properties and we only having 3 listed on the database.

Also note that the PropertyName must match the property name of the object that I’ll use for the dynamic assignment.

Building the object

Now that we have the database table, we need to build the repository to get that and be able to use them in the code.

I will not be making a tutorial of this part, I’ll skip right at the start of the new GetExcelBytes() function.

public static byte[] GetExcelBytes(List<string> columns) {
    List<ExcelItem> excelItems = new List<ExcelItem>();
    List<dynamic> objectsToExcel = new List<dynamic>();
    ExcelExportService exportService = new ExcelExportService();

    foreach (var item in itemsToExcel)
        dynamic newObject = new ExpandoObject();
        foreach (var col in columns)
            this.AddProperty(newObject, col, product.GetType().GetProperty(col).GetValue(item, null));


    return exportService.ExportToExcel(objectsToExcel);

Now we will be having creating an object with some of the properties of ExcelItem, but completely dynamic. Instead of using all the 6 properties, we are just using 3 of them, the one that we only want to send.

Now let’s pretend that this ExcelItem object has 200 properties, crazy, but it could happen.

The only thing that I have to do is to insert those properties that I want to render into the Excel file to that ColumnExcelItem table and that’s it.


In this case we just used a single case, but let’s say that you want to have different reports for some users. Let’s say that the admin role should get all the properties, then you would create something like this structure in the database


Id    PropertyName
1     Admin
2     Users

Id    TemplateId    PropertyName
1     1             Id
2     1             Name
3     1             Surname
4     1             Age
5     1             CreatedAt
6     1             UpdatedAt
7     2             Name
8     2             Surname
9     2             Age

Then by getting the template you could have different columns, all dynamically and no code related.

If we have an user with the role Admins, the file will contain columns Id, Name, Surname, Age, CreatedAt, UpdatedAt.

In the case if you generate it using the Users role, it will have Name, Age. Surname.


This is a cool way of learing how the dynamic works in .NET, and it’s a really good solution when you need to have different roles or templates for different users, and not really interested on investing time hardcoding stuff.