Why does Boolean.ToString output "True" and not "true"

Only people from Microsoft can really answer that question. However, I'd like to offer some fun facts about it ;)

First, this is what it says in MSDN about the Boolean.ToString() method:

Return Value

Type: System.String

TrueString if the value of this instance is true, or FalseString if the value of this instance is false.

Remarks

This method returns the constants "True" or "False". Note that XML is case-sensitive, and that the XML specification recognizes "true" and "false" as the valid set of Boolean values. If the String object returned by the ToString() method is to be written to an XML file, its String.ToLower method should be called first to convert it to lowercase.

Here comes the fun fact #1: it doesn't return TrueString or FalseString at all. It uses hardcoded literals "True" and "False". Wouldn't do you any good if it used the fields, because they're marked as readonly, so there's no changing them.

The alternative method, Boolean.ToString(IFormatProvider) is even funnier:

Remarks

The provider parameter is reserved. It does not participate in the execution of this method. This means that the Boolean.ToString(IFormatProvider) method, unlike most methods with a provider parameter, does not reflect culture-specific settings.

What's the solution? Depends on what exactly you're trying to do. Whatever it is, I bet it will require a hack ;)

...because the .NET environment is designed to support many languages.

System.Boolean (in mscorlib.dll) is designed to be used internally by languages to support a boolean datatype. C# uses all lowercase for its keywords, hence 'bool', 'true', and 'false'.

VB.NET however uses standard casing: hence 'Boolean', 'True', and 'False'.

Since the languages have to work together, you couldn't have true.ToString() (C#) giving a different result to True.ToString() (VB.NET). The CLR designers picked the standard CLR casing notation for the ToString() result.

The string representation of the boolean true is defined to be Boolean.TrueString.

(There's a similar case with System.String: C# presents it as the 'string' type).