Error using structs in calling unmanaged dll



I don't understand the following error I'm getting when I call the DoOpen
function of my unmanaged dll. Can someone explain please

[DllImport(DllName, CharSet=charset)]
public static extern int DoOpen(ref sRDetails RDetails);

"An unhandled exception of type 'System.TypeLoadException' occurred in
Additional information: Can not marshal field ChDetails of type sRDetails:
This type can not be marshaled as a structure field."

public struct sRDetails
public int UniqueID;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAXSTR_SNAME)]
public string SName;
public sTM TStamp;
public int ChEntries;
[MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_CH)]
public sChDetails[] ChDetails;

public struct sChDetails
public sHCode HCode;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAXSTR_CH_NAME)]
public string ChName;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAXSTR_UNITS)]
public string Units1;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAXSTR_UNITS)]
public string Units2;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAXSTR_HLLIMIT)]
public string HLLimit;
public float Value;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAXSTR_TRSTATE)]
public string TrState;


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