How to check if one value of an object is null

You can gather all your strings into an array and then run .Any() method:

if (new[] { obj.string1, obj.string2, obj.string3 }.Any(string.IsNullOrWhiteSpace))
{
    
}

Alternatively you can use reflection (which will affect your code's performance), to scan all the strings of your object and check your condition:

var anyEmpty = obj.GetType().GetProperties()
    .Any(x => x.PropertyType == typeof(string)
              && string.IsNullOrWhiteSpace(x.GetValue(obj) as string));

You can use a for loop to iterate trough all strings and check if they are blank or empty.

EDIT: You probably have to add all the strings into an array or a list, because they all have different names like string1, string2 and string3

If you do this a lot, you could write a method to check it:

public static class Ensure
{
    public static bool NoneNullOrWhitespace(params string?[] items)
    {
        return !items.Any(string.IsNullOrWhiteSpace);
    }
}

Which for your case you would call like this:

if (Ensure.NoneNullOrWhitespace(object.string1, object.string2, object.string3))
{
    ...
}

If it's an option for you to define a class for your objects, you could let the class itself handle the "all strings not null or whitespace"-check:

public class MyObject
{
    public string String1 { get; set; }
    public string String2 { get; set; }
    public string String3 { get; set; }

    public bool StringsAreNotNullOrWhiteSpace => !Strings.Any(string.IsNullOrWhiteSpace);

    private string[] Strings => new[] { String1, String2, String3 };
}

and use it like this:

var myObject = new MyObject();
//Populate myObject

if (myObject.StringsAreNotNullOrWhiteSpace)
{
    //Add myObject to list
}

(The implementation of StringsAreNotNullOrWhiteSpace is basically what @mickl did in their first suggestion, but returning the opposite bool value.)