方法一：利用System.IO.DriveInfo.GetDrives方法来获取

///   
        /// 获取指定驱动器的空间总大小(单位为B) 
        ///   
        ///  只需输入代表驱动器的字母即可 （大写） 
        ///    
        public static long GetHardDiskSpace(string str_HardDiskName) 
        { 
            long totalSize= new long(); 
            str_HardDiskName=str_HardDiskName +":\\"; 
            System.IO.DriveInfo[] drives = System.IO.DriveInfo.GetDrives(); 
            foreach (System.IO.DriveInfo drive in drives) 
            { 
                if (drive.Name == str_HardDiskName) 
                {
                    totalSize = drive.TotalSize / (1024 * 1024 * 1024); 
                } 
            } 
            return totalSize; 
        }

        ///   
        /// 获取指定驱动器的剩余空间总大小(单位为B) 
        ///   
        ///  只需输入代表驱动器的字母即可  
        ///    
        public static long GetHardDiskFreeSpace(string str_HardDiskName) 
        { 
            long freeSpace = new long(); 
            str_HardDiskName = str_HardDiskName + ":\\"; 
            System.IO.DriveInfo[] drives = System.IO.DriveInfo.GetDrives(); 
            foreach (System.IO.DriveInfo drive in drives) 
            { 
                if (drive.Name == str_HardDiskName) 
                {
                    freeSpace = drive.TotalFreeSpace / (1024 * 1024 * 1024); 
                } 
            } 
            return freeSpace; 
        }

方法二：利用ManagementClass("Win32_LogicalDisk")来获取

List<Dictionary<string, string>> diskInfoDic = new List<Dictionary<string, string>>();
ManagementClass diskClass = new ManagementClass("Win32_LogicalDisk");
			ManagementObjectCollection disks = diskClass.GetInstances();
			foreach(ManagementObject disk in disks)
			{
				Dictionary<string, string> diskInfo = new Dictionary<string, string>();
				try
				{
					// 磁盘名称
					diskInfo["Name"] =disk["Name"].ToString();
					// 磁盘描述
					diskInfo["Description"]=disk["Description"].ToString();
					// 磁盘总容量，可用空间，已用空间
					if (System.Convert.ToInt64(disk["Size"]) > 0)
					{
						long totalSpace = System.Convert.ToInt64(disk["Size"]) / MB;
						long freeSpace = System.Convert.ToInt64(disk["FreeSpace"]) / MB;
						long usedSpace = totalSpace - freeSpace;
                        			diskInfo["totalSpace"]=totalSpace.ToString();
						diskInfo["usedSpace"]=usedSpace.ToString();
						diskInfo["freeSpace"]=freeSpace.ToString();
					}
					diskInfoDic.Add(diskInfo);
				}
				catch(Exception ex)
				{
					Throw ex;
				}
			}

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C# 获取磁盘空间大小的方法