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In the documentation you can read the following for CultureInfo

The CultureInfo class holds culture-specific information, such as the associated language, sublanguage, country/region, calendar, and cultural conventions. This class also provides access to culture-specific instances of DateTimeFormatInfo, NumberFormatInfo, CompareInfo, and TextInfo. These objects contain the information required for culture-specific operations, such as casing, formatting dates and numbers, and comparing strings.

First i’ll initialize an instance of a customized CultureInfo and install it in the current thread:

CultureInfo appCultureInfo = new CultureInfo("en-US");
appCultureInfo.NumberFormat.NumberDecimalSeparator = ".";
appCultureInfo.NumberFormat.NumberGroupSeparator = " ";
appCultureInfo.NumberFormat.NumberDecimalDigits = 2;
appCultureInfo.NumberFormat.CurrencySymbol = "€";
Thread.CurrentThread.CurrentCulture = appCultureInfo;

Note: The culture in Thread.CurrentThread.CurrentUICulture is only used by the Resource Manager to lookup culture-specific resources at run-time. Since we’re not playing with resources, we don’t have to care about this one.

If you don’t provide the desired format specifier, you will get the generic format specifier “G” ( Standard Numeric, DateTime, Enumeration Format Strings) And now it’s time for a little demo

public double SomeValue
{
	get { return 12345.6789; }
}

private void Form1_Load(object sender, EventArgs e)
{
	label1.Text = "default to generic format: " + SomeValue.ToString();
	label2.Text = "numeric format: " + SomeValue.ToString("N");
	label3.Text = String.Format("currency format: {0:C}", SomeValue);
	label4.DataBindings.Add("Text", this, "SomeValue", true, DataSourceUpdateMode.Never, string.Empty, "P");

	this.Column1.DefaultCellStyle.Format = "E1";
	this.Column1.DataPropertyName = "SomeValue";
	this.dataGridView1.AutoGenerateColumns = false;
	this.dataGridView1.DataSource = new Form1[] { this };
}

Another important interface for formatting is IFormattable which provides functionality to format the value of an object into a string representation. Put simply, it allows you to define your own format specifiers. Here is an example that allows the user to build his own representation of a Person

class Person : IFormattable
{
	// omitted the code for 4 properties: Id, Name, Title and Birthday

	public override string ToString()
	{
		return ToString("{Id}, {Name}", null);
	}

	public string ToString(string format, IFormatProvider formatProvider)
	{
		string actualFormat = format;

		string[] replacements = new string[] { "Id", "Name", "Title", "Birthday" };
		for (int i = 0; i < replacements.Length; ++i) 
		{ 
			actualFormat = actualFormat.Replace(replacements[i], i.ToString()); 
		} 
		return string.Format(actualFormat, this.id, this.name, this.title, this.birthday); 
	} 
} 

private void Form1_Load(object sender, EventArgs e) 
{ 
	List<person> persons = new List<person>();
	persons.Add(new Person(1, new DateTime(1980, 4, 30), "Tim", "Sir"));
	persons.Add(new Person(2, new DateTime(1974, 4, 9), "Jenna", "Miss"));

	this.comboBox1.FormatString = "{Name} born on {Birthday}";
	this.comboBox1.FormattingEnabled = true;
	this.comboBox1.DataSource = persons;
}