When treating cultures, symbols and unites NumberFormatInfo.InvariantInfo is an easy way to get the Invariant culture. This instance however is read-only and can’t be modified. If you attempt to modify it you will obtain System.InvalidOperationException: Instance is read-only exception. This took me by surprise and caused my unit tests to fail and so I dug a bit deeper into the issue to understand how the read-only instance is created.

The following method was causing the issue:

public String GetBalance(String globalBalance){
  NumberFormatInfo formatInfo = NumberFormatInfo.InvariantInfo;
  formatInfo.CurrencySymbol = "$";
  return globalBalance.ToString("C", formatInfo);
}

After looking at my code it was clear that this is buggy. I was trying to change the InvariantFormat which surely was not allowed. The correct way to do things would be to create new culture based on the current cultures thread name.

I decided to check to look how such readonly instance is implemented in .NET. The stack trace gives a bit of info where to look.

at System.Globalization.NumberFormatInfo.VerifyWritable()
   at System.Globalization.NumberFormatInfo.set_CurrencySymbol(String value)
   at

Lets take a look at the source code of the NumberFormatInfo class and search for the CurrencySymbol property, for example at this site

public String CurrencySymbol {
  get { return currencySymbol; }
  set {
    VerifyWritable();
    if (value == null) {
      throw new ArgumentNullException("CurrencySymbol",
      Environment.GetResourceString("ArgumentNull_String"));
    }
    currencySymbol = value;
  }
}

internal bool isReadOnly=false;

private void VerifyWritable() {
  if (isReadOnly) {
    throw new
InvalidOperationException(Environment.GetResourceString("InvalidOperation_ReadOnly"));
  }
}

public static NumberFormatInfo ReadOnly(NumberFormatInfo nfi) {
  if (nfi == null) {
    throw new ArgumentNullException("nfi");
  }
  if (nfi.IsReadOnly) {
    return (nfi);
  }
  NumberFormatInfo info = (NumberFormatInfo)(nfi.MemberwiseClone());
  info.isReadOnly = true;
  return info;
}

Ok so any time the property is set (any of the properties of NumberInfoFormat) the VerifyWritable method is called. And this method just checks the private field isReadOnly. This makes sense. This property is set up by the ReadOnly static method. So any time you will need a read only instance of NumberFormatInfo you can just call:

NumberFormatInfo myReadOnlyInstance = NumberFormatInfo.ReadOnly(formatInfo);