An inelegant way would be:

var my = DoFormat(123.0);

With DoFormat being something like:

public static string DoFormat( double myNumber )
{
    var s = string.Format("{0:0.00}", myNumber);

    if ( s.EndsWith("00") )
    {
        return ((int)myNumber).ToString();
    }
    else
    {
        return s;
    }
}

Not elegant but working for me in similar situations in some projects.

There are different overloads for decimal.ToString based on what formatting you want.

Example

decimal d = 5.00
Console.WriteLine(d.ToString("C")); // for currency

See below for other overloads... specifier is what you put into the ToString(specifier)

MSDN Documentation on Decimal.ToString

decimal value = 16325.62m; string specifier;

// Use standard numeric format specifiers.
specifier = "G";
Console.WriteLine("{0}: {1}", specifier, value.ToString(specifier));
// Displays:    G: 16325.62
specifier = "C";
Console.WriteLine("{0}: {1}", specifier, value.ToString(specifier));
// Displays:    C: $16,325.62
specifier = "E04";
Console.WriteLine("{0}: {1}", specifier, value.ToString(specifier));
// Displays:    E04: 1.6326E+004
specifier = "F";
Console.WriteLine("{0}: {1}", specifier, value.ToString(specifier));
// Displays:    F: 16325.62
specifier = "N";
Console.WriteLine("{0}: {1}", specifier, value.ToString(specifier));
// Displays:    N: 16,325.62
specifier = "P";
Console.WriteLine("{0}: {1}", specifier, (value/10000).ToString(specifier));
// Displays:    P: 163.26 %

// Use custom numeric format specifiers.
specifier = "0,0.000";
Console.WriteLine("{0}: {1}", specifier, value.ToString(specifier));
// Displays:    0,0.000: 16,325.620
specifier = "#,#.00#;(#,#.00#)";
Console.WriteLine("{0}: {1}", specifier, (value*-1).ToString(specifier));
// Displays:    #,#.00#;(#,#.00#): (16,325.62)

The following extension method should satisfy you requirements. The comments on the code was provided in OP and comments.

public static string ToFormattedString(this decimal d)
{
    //The comma is not mandatory. but 
    var s = d.ToString();
    var tokens = s.Split(new[]{"."}, StringSplitOptions.RemoveEmptyEntries);
    //if there are no decimal points 12 then there should no be any zeros and periods (.)
    if (tokens.Length == 1)
        return s;
    //I need to remove trailing zeros
    var places = tokens[1].TrimEnd('0');
    if (places.Length == 0)
        return tokens[0];
    //if there is only one decimal point ex- 0.5 then it should be displayed as 0.50 
    if (places.Length == 1)
        return d.ToString("F2");
    var format = string.Format("F{0}", places.Length);
    return d.ToString(format);
}

Used like this

var x = new decimal[]{120.5m, 110, 25.356m};
foreach (var item in x)
    Console.WriteLine("{0} => {1}", item.ToString(), item.ToFormattedString());

Output:

120.5 => 120.50
110 => 110
25.356 => 25.356

You want java.text.DecimalFormat.

DecimalFormat df = new DecimalFormat("0.00##");
String result = df.format(34.4959);

Use the string format function:

>>> from decimal import Decimal
>>> d = Decimal("0.0000000000000123123")
>>> s = '{0:f}'.format(d)
>>> print(s)
0.0000000000000123123

If you just type cast the number to a string it won't work for exponents:

>>> str(d)
'1.23123E-14' 

This should work, if its until 2 decimals:

Decimal dec;
String amount;
if (!string.valueof(dec.format()).right(3).contains('.')){
    amount = '$' + string.valueof(dec.format()) + '.00';
}else if (string.valueof(dec.format()).right(2).contains('.')){
    amount = '$' + string.valueof(dec.format()) + '0';
}else {
    amount = '$' + string.valueof(dec.format());
}