[Solved] Build the string dynamically based on the length in c#

Question

Firstly define an extension method for StringBuilder:

public static class StringBuilderExtensions
{
    public static void AppendPadded(this StringBuilder builder, string value, int length);
    {
        builder.Append($"{value}{new string(' ', length)}".Substring(0, length));
    }
    public static void AppendPadded(this StringBuilder builder, int value, int length);
    {
        builder.Append($"{new string('0', length)}{value}".Reverse().ToString().Substring(0, length).Reverse().ToString());
    }
}

Then use:

StringBuilder builder = new StringBuilder();
builder.AppendPadded("Hiwor", 8);
builder.AppendPadded("meena", 10);
return builder.ToString();

Or with your example:

foreach (string item in list)
    builder.AppendPadded(item, transLine[item]);

EDIT: Ok, so looks like you want to be able to define a format then build the string using it. Try:

(you will need to reference System.ComponentModel.DataAnnotations and System.Reflection for this)

public abstract class AnItem
{
    private static int GetLength(PropertyInfo property)
    {
        StringLengthAttribute attribute = property.GetCustomAttributes(typeof(StringLengthAttribute), true).FirstOrDefault() as StringLengthAttribute;
        if (attribute == null)
            throw new Exception($"StringLength not specified for {property.Name}");
        return attribute.MaxLength();
    }
    private string GetPropertyValue(PropertyInfo property)
    {
        if (property.PropertyType == typeof(string))
            return property.GetValue(this);
        else if (property.PropertyType == typeof(int))
            return property.GetValue(this).ToString();
        else
            throw new Exception($"Property '{property.Name}' is not a supported type");
    }
    private static void SetPropertyValue(object item, PropertyInfo property, string value)
    {
        if (property.PropertyType == typeof(string))
            property.SetValue(item, value, null);
        else if (property.PropertyType == typeof(int))
            property.SetValue(item, int.Parse(value), null);
        else
            throw new Exception($"Property '{property.Name}' is not a supported type");
    }
    public string GetPaddedString()
    {
        StringBuilder builder = new StringBuilder();
        PropertyInfo[] properties = GetType().GetProperties();
        foreach (PropertyInfo property in properties)
            builder.AppendPadded(GetPropertyValue(property), GetLength(property));
        return builder.ToString();
    }
    public static T CreateFromPaddedString<T>(string paddedString) where T : AnItem, new()
    {
        T item = new T();
        int offset = 0;
        PropertyInfo[] properties = typeof(T).GetProperties();
        foreach (PropertyInfo property in properties)
        {
            int length = GetLength(property);
            if (offset + length > paddedString.Length)
                throw new Exception("The string is too short");
            SetPropertyValue(item, property, paddedString.Substring(offset, length)));
            offset += length;
        }
        if (offset < paddedString.Length)
            throw new Exception("The string is too long");
        return item;
    }
}
public class MyItem : AnItem
{
    [StringLength(1)]
    public string ProductCode { get; set; }

    [StringLength(12)]
    public string ApplicantFirstName { get; set; }

    [StringLength(1)]
    public string ApplicantMiddleInitial { get; set; }

    [StringLength(1)]
    public string Partner { get; set; }

    [StringLength(8)]
    public string Employee { get; set; }
}

Then use it:

MyItem item = new MyItem
{
    ProductCode = "grouplife",
    ApplicantFirstName = "meena",
    ApplicantMiddleInitial = "d",
    Partner = "yes",
    Employee = "yes"
};

string paddedString = item.GetPaddedString();

And to read a string to get an item:

MyItem item = AnItem.CreateFromPaddedString<MyItem>(paddedString);

Accepted Answer