[MonoDevelop] compiler error message about 'string'

[=?gb2312?B?1tPS4w==?=]

version info:
monodevelop-0.12-7.fc6



The source

1532 if (!String.IsNullOrEmpty(entry))



The message:

1532 'string' does not contain a definition for 'IsNullOrEmpty'(CS0117)


Also the strange is that, when I try to modify 'String' in the source to 'string', the compiler still bump out the same message, but the line number changed to 0 as,

0 'string' does not contain a definition for 'IsNullOrEmpty'(CS0117)


Another problem, when I compiler codes use CSharpCodeProvider, got similar message as,

'Microsoft CSharp.CSharpCodeProvider' does not contain a definition for 'CompileAssemblyFromSource'(CS0117)


thanks
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