Most of the times, we need to interact with native windows actions in WPF or Win Forms. C# provides a better way to import an assembly dynamically and we can effectively use the methods of native System assembly. 

        class Example
        {
            // Use DllImport to import the Win32 MessageBox function.
            [DllImport("user32.dll", CharSet = CharSet.Unicode)]
            public static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);

            static void Main()
            {
                // Call the MessageBox function using platform invoke.
                MessageBox(new IntPtr(0), "Hello World!", "Hello Dialog", 0);
            }
        }

One of the most useful assembly if user32 assembly which contains the 732 methods.

ActivateKeyboardLayout 1
AdjustWindowRect 2
AdjustWindowRectEx 3
AlignRects 4
AllowForegroundActivation 5
AllowSetForegroundWindow 6
AnimateWindow 7
AnyPopup 8
AppendMenuA 9
AppendMenuW 10
ArrangeIconicWindows 11
AttachThreadInput 12
BeginDeferWindowPos 13
BeginPaint 14
BlockInput 15
BringWindowToTop 16
BroadcastSystemMessage 17
BroadcastSystemMessageA 18
BroadcastSystemMessageExA 19
BroadcastSystemMessageExW 20
BroadcastSystemMessageW 21
BuildReasonArray 22
CalcMenuBar 23
CallMsgFilter 24
CallMsgFilterA 25
CallMsgFilterW 26
CallNextHookEx 27
CallWindowProcA 28
CallWindowProcW 29
CascadeChildWindows 30
CascadeWindows 31
ChangeClipboardChain 32
ChangeDisplaySettingsA 33
ChangeDisplaySettingsExA 34
ChangeDisplaySettingsExW 35
ChangeDisplaySettingsW 36
ChangeMenuA 37
ChangeMenuW 38
CharLowerA 39
CharLowerBuffA 40
CharLowerBuffW 41
CharLowerW 42
CharNextA 43
CharNextExA 44
CharNextW 45
CharPrevA 46
CharPrevExA 47
CharPrevW 48
CharToOemA 49
CharToOemBuffA 50
CharToOemBuffW 51
CharToOemW 52
CharUpperA 53
CharUpperBuffA 54
CharUpperBuffW 55
CharUpperW 56
CheckDlgButton 57
CheckMenuItem 58
CheckMenuRadioItem 59
CheckRadioButton 60
ChildWindowFromPoint 61
ChildWindowFromPointEx 62
CliImmSetHotKey 63
ClientThreadSetup 64
ClientToScreen 65
ClipCursor 66
CloseClipboard 67
CloseDesktop 68
CloseWindow 69
CloseWindowStation 70
CopyAcceleratorTableA 71
CopyAcceleratorTableW 72
CopyIcon 73
CopyImage 74
CopyRect 75
CountClipboardFormats 76
CreateAcceleratorTableA 77
CreateAcceleratorTableW 78
CreateCaret 79
CreateCursor 80
CreateDesktopA 81
CreateDesktopW 82
CreateDialogIndirectParamA 83
CreateDialogIndirectParamAorW 84
CreateDialogIndirectParamW 85
CreateDialogParamA 86
CreateDialogParamW 87
CreateIcon 88
CreateIconFromResource 89
CreateIconFromResourceEx 90
CreateIconIndirect 91
CreateMDIWindowA 92
CreateMDIWindowW 93
CreateMenu 94
CreatePopupMenu 95
CreateSystemThreads 96
CreateWindowExA 97
CreateWindowExW 98
CreateWindowStationA 99
CreateWindowStationW 100
CsrBroadcastSystemMessageExW 101
CtxInitUser32 102
DdeAbandonTransaction 103
DdeAccessData 104
DdeAddData 105
DdeClientTransaction 106
DdeCmpStringHandles 107
DdeConnect 108
DdeConnectList 109
DdeCreateDataHandle 110
DdeCreateStringHandleA 111
DdeCreateStringHandleW 112
DdeDisconnect 113
DdeDisconnectList 114
DdeEnableCallback 115
DdeFreeDataHandle 116
DdeFreeStringHandle 117
DdeGetData 118
DdeGetLastError 119
DdeGetQualityOfService 120
DdeImpersonateClient 121
DdeInitializeA 122
DdeInitializeW 123
DdeKeepStringHandle 124
DdeNameService 125
DdePostAdvise 126
DdeQueryConvInfo 127
DdeQueryNextServer 128
DdeQueryStringA 129
DdeQueryStringW 130
DdeReconnect 131
DdeSetQualityOfService 132
DdeSetUserHandle 133
DdeUnaccessData 134
DdeUninitialize 135
DefDlgProcA 136
DefDlgProcW 137
DefFrameProcA 138
DefFrameProcW 139
DefMDIChildProcA 140
DefMDIChildProcW 141
DefRawInputProc 142
DefWindowProcA 143
DefWindowProcW 144
DeferWindowPos 145
DeleteMenu 146
DeregisterShellHookWindow 147
DestroyAcceleratorTable 148
DestroyCaret 149
DestroyCursor 150
DestroyIcon 151
DestroyMenu 152
DestroyReasons 153
DestroyWindow 154
DeviceEventWorker 155
DialogBoxIndirectParamA 156
DialogBoxIndirectParamAorW 157
DialogBoxIndirectParamW 158
DialogBoxParamA 159
DialogBoxParamW 160
DisableProcessWindowsGhosting 161
DispatchMessageA 162
DispatchMessageW 163
DisplayExitWindowsWarnings 164
DlgDirListA 165
DlgDirListComboBoxA 166
DlgDirListComboBoxW 167
DlgDirListW 168
DlgDirSelectComboBoxExA 169
DlgDirSelectComboBoxExW 170
DlgDirSelectExA 171
DlgDirSelectExW 172
DragDetect 173
DragObject 174
DrawAnimatedRects 175
DrawCaption 176
DrawCaptionTempA 177
DrawCaptionTempW 178
DrawEdge 179
DrawFocusRect 180
DrawFrame 181
DrawFrameControl 182
DrawIcon 183
DrawIconEx 184
DrawMenuBar 185
DrawMenuBarTemp 186
DrawStateA 187
DrawStateW 188
DrawTextA 189
DrawTextExA 190
DrawTextExW 191
DrawTextW 192
EditWndProc 193
EmptyClipboard 194
EnableMenuItem 195
EnableScrollBar 196
EnableWindow 197
EndDeferWindowPos 198
EndDialog 199
EndMenu 200
EndPaint 201
EndTask 202
EnterReaderModeHelper 203
EnumChildWindows 204
EnumClipboardFormats 205
EnumDesktopWindows 206
EnumDesktopsA 207
EnumDesktopsW 208
EnumDisplayDevicesA 209
EnumDisplayDevicesW 210
EnumDisplayMonitors 211
EnumDisplaySettingsA 212
EnumDisplaySettingsExA 213
EnumDisplaySettingsExW 214
EnumDisplaySettingsW 215
EnumPropsA 216
EnumPropsExA 217
EnumPropsExW 218
EnumPropsW 219
EnumThreadWindows 220
EnumWindowStationsA 221
EnumWindowStationsW 222
EnumWindows 223
EqualRect 224
ExcludeUpdateRgn 225
ExitWindowsEx 226
FillRect 227
FindWindowA 228
FindWindowExA 229
FindWindowExW 230
FindWindowW 231
FlashWindow 232
FlashWindowEx 233
FrameRect 234
FreeDDElParam 235
GetActiveWindow 236
GetAltTabInfo 237
GetAltTabInfoA 238
GetAltTabInfoW 239
GetAncestor 240
GetAppCompatFlags 242
GetAppCompatFlags2 241
GetAsyncKeyState 243
GetCapture 244
GetCaretBlinkTime 245
GetCaretPos 246
GetClassInfoA 247
GetClassInfoExA 248
GetClassInfoExW 249
GetClassInfoW 250
GetClassLongA 251
GetClassLongW 252
GetClassNameA 253
GetClassNameW 254
GetClassWord 255
GetClientRect 256
GetClipCursor 257
GetClipboardData 258
GetClipboardFormatNameA 259
GetClipboardFormatNameW 260
GetClipboardOwner 261
GetClipboardSequenceNumber 262
GetClipboardViewer 263
GetComboBoxInfo 264
GetCursor 265
GetCursorFrameInfo 266
GetCursorInfo 267
GetCursorPos 268
GetDC 269
GetDCEx 270
GetDesktopWindow 271
GetDialogBaseUnits 272
GetDlgCtrlID 273
GetDlgItem 274
GetDlgItemInt 275
GetDlgItemTextA 276
GetDlgItemTextW 277
GetDoubleClickTime 278
GetFocus 279
GetForegroundWindow 280
GetGUIThreadInfo 281
GetGuiResources 282
GetIconInfo 283
GetInputDesktop 284
GetInputState 285
GetInternalWindowPos 286
GetKBCodePage 287
GetKeyNameTextA 288
GetKeyNameTextW 289
GetKeyState 290
GetKeyboardLayout 291
GetKeyboardLayoutList 292
GetKeyboardLayoutNameA 293
GetKeyboardLayoutNameW 294
GetKeyboardState 295
GetKeyboardType 296
GetLastActivePopup 297
GetLastInputInfo 298
GetLayeredWindowAttributes 299
GetListBoxInfo 300
GetMenu 301
GetMenuBarInfo 302
GetMenuCheckMarkDimensions 303
GetMenuContextHelpId 304
GetMenuDefaultItem 305
GetMenuInfo 306
GetMenuItemCount 307
GetMenuItemID 308
GetMenuItemInfoA 309
GetMenuItemInfoW 310
GetMenuItemRect 311
GetMenuState 312
GetMenuStringA 313
GetMenuStringW 314
GetMessageA 315
GetMessageExtraInfo 316
GetMessagePos 317
GetMessageTime 318
GetMessageW 319
GetMonitorInfoA 320
GetMonitorInfoW 321
GetMouseMovePointsEx 322
GetNextDlgGroupItem 323
GetNextDlgTabItem 324
GetOpenClipboardWindow 325
GetParent 326
GetPriorityClipboardFormat 327
GetProcessDefaultLayout 328
GetProcessWindowStation 329
GetProgmanWindow 330
GetPropA 331
GetPropW 332
GetQueueStatus 333
GetRawInputBuffer 334
GetRawInputData 335
GetRawInputDeviceInfoA 336
GetRawInputDeviceInfoW 337
GetRawInputDeviceList 338
GetReasonTitleFromReasonCode 339
GetRegisteredRawInputDevices 340
GetScrollBarInfo 341
GetScrollInfo 342
GetScrollPos 343
GetScrollRange 344
GetShellWindow 345
GetSubMenu 346
GetSysColor 347
GetSysColorBrush 348
GetSystemMenu 349
GetSystemMetrics 350
GetTabbedTextExtentA 351
GetTabbedTextExtentW 352
GetTaskmanWindow 353
GetThreadDesktop 354
GetTitleBarInfo 355
GetTopWindow 356
GetUpdateRect 357
GetUpdateRgn 358
GetUserObjectInformationA 359
GetUserObjectInformationW 360
GetUserObjectSecurity 361
GetWinStationInfo 362
GetWindow 363
GetWindowContextHelpId 364
GetWindowDC 365
GetWindowInfo 366
GetWindowLongA 367
GetWindowLongW 368
GetWindowModuleFileName 369
GetWindowModuleFileNameA 370
GetWindowModuleFileNameW 371
GetWindowPlacement 372
GetWindowRect 373
GetWindowRgn 374
GetWindowRgnBox 375
GetWindowTextA 376
GetWindowTextLengthA 377
GetWindowTextLengthW 378
GetWindowTextW 379
GetWindowThreadProcessId 380
GetWindowWord 381
GrayStringA 382
GrayStringW 383
HideCaret 384
HiliteMenuItem 385
IMPGetIMEA 386
IMPGetIMEW 387
IMPQueryIMEA 388
IMPQueryIMEW 389
IMPSetIMEA 390
IMPSetIMEW 391
ImpersonateDdeClientWindow 392
InSendMessage 393
InSendMessageEx 394
InflateRect 395
InitializeLpkHooks 396
InitializeWin32EntryTable 397
InsertMenuA 398
InsertMenuItemA 399
InsertMenuItemW 400
InsertMenuW 401
InternalGetWindowText 402
IntersectRect 403
InvalidateRect 404
InvalidateRgn 405
InvertRect 406
IsCharAlphaA 407
IsCharAlphaNumericA 408
IsCharAlphaNumericW 409
IsCharAlphaW 410
IsCharLowerA 411
IsCharLowerW 412
IsCharUpperA 413
IsCharUpperW 414
IsChild 415
IsClipboardFormatAvailable 416
IsDialogMessage 417
IsDialogMessageA 418
IsDialogMessageW 419
IsDlgButtonChecked 420
IsGUIThread 421
IsHungAppWindow 422
IsIconic 423
IsMenu 424
IsRectEmpty 425
IsServerSideWindow 426
IsWinEventHookInstalled 427
IsWindow 428
IsWindowEnabled 429
IsWindowInDestroy 430
IsWindowUnicode 431
IsWindowVisible 432
IsZoomed 433
KillSystemTimer 434
KillTimer 435
LoadAcceleratorsA 436
LoadAcceleratorsW 437
LoadBitmapA 438
LoadBitmapW 439
LoadCursorA 440
LoadCursorFromFileA 441
LoadCursorFromFileW 442
LoadCursorW 443
LoadIconA 444
LoadIconW 445
LoadImageA 446
LoadImageW 447
LoadKeyboardLayoutA 448
LoadKeyboardLayoutEx 449
LoadKeyboardLayoutW 450
LoadLocalFonts 451
LoadMenuA 452
LoadMenuIndirectA 453
LoadMenuIndirectW 454
LoadMenuW 455
LoadRemoteFonts 456
LoadStringA 457
LoadStringW 458
LockSetForegroundWindow 459
LockWindowStation 460
LockWindowUpdate 461
LockWorkStation 462
LookupIconIdFromDirectory 463
LookupIconIdFromDirectoryEx 464
MBToWCSEx 465
MB_GetString 466
MapDialogRect 467
MapVirtualKeyA 468
MapVirtualKeyExA 469
MapVirtualKeyExW 470
MapVirtualKeyW 471
MapWindowPoints 472
MenuItemFromPoint 473
MenuWindowProcA 474
MenuWindowProcW 475
MessageBeep 476
MessageBoxA 477
MessageBoxExA 478
MessageBoxExW 479
MessageBoxIndirectA 480
MessageBoxIndirectW 481
MessageBoxTimeoutA 482
MessageBoxTimeoutW 483
MessageBoxW 484
ModifyMenuA 485
ModifyMenuW 486
MonitorFromPoint 487
MonitorFromRect 488
MonitorFromWindow 489
MoveWindow 490
MsgWaitForMultipleObjects 491
MsgWaitForMultipleObjectsEx 492
NotifyWinEvent 493
OemKeyScan 494
OemToCharA 495
OemToCharBuffA 496
OemToCharBuffW 497
OemToCharW 498
OffsetRect 499
OpenClipboard 500
OpenDesktopA 501
OpenDesktopW 502
OpenIcon 503
OpenInputDesktop 504
OpenWindowStationA 505
OpenWindowStationW 506
PackDDElParam 507
PaintDesktop 508
PaintMenuBar 509
PeekMessageA 510
PeekMessageW 511
PostMessageA 512
PostMessageW 513
PostQuitMessage 514
PostThreadMessageA 515
PostThreadMessageW 516
PrintWindow 517
PrivateExtractIconExA 518
PrivateExtractIconExW 519
PrivateExtractIconsA 520
PrivateExtractIconsW 521
PrivateSetDbgTag 522
PrivateSetRipFlags 523
PtInRect 524
QuerySendMessage 525
QueryUserCounters 526
RealChildWindowFromPoint 527
RealGetWindowClass 528
RealGetWindowClassA 529
RealGetWindowClassW 530
ReasonCodeNeedsBugID 531
ReasonCodeNeedsComment 532
RecordShutdownReason 533
RedrawWindow 534
RegisterClassA 535
RegisterClassExA 536
RegisterClassExW 537
RegisterClassW 538
RegisterClipboardFormatA 539
RegisterClipboardFormatW 540
RegisterDeviceNotificationA 541
RegisterDeviceNotificationW 542
RegisterHotKey 543
RegisterLogonProcess 544
RegisterMessagePumpHook 545
RegisterRawInputDevices 546
RegisterServicesProcess 547
RegisterShellHookWindow 548
RegisterSystemThread 549
RegisterTasklist 550
RegisterUserApiHook 551
RegisterWindowMessageA 552
RegisterWindowMessageW 553
ReleaseCapture 554
ReleaseDC 555
RemoveMenu 556
RemovePropA 557
RemovePropW 558
ReplyMessage 559
ResolveDesktopForWOW 560
ReuseDDElParam 561
ScreenToClient 562
ScrollChildren 563
ScrollDC 564
ScrollWindow 565
ScrollWindowEx 566
SendDlgItemMessageA 567
SendDlgItemMessageW 568
SendIMEMessageExA 569
SendIMEMessageExW 570
SendInput 571
SendMessageA 572
SendMessageCallbackA 573
SendMessageCallbackW 574
SendMessageTimeoutA 575
SendMessageTimeoutW 576
SendMessageW 577
SendNotifyMessageA 578
SendNotifyMessageW 579
SetActiveWindow 580
SetCapture 581
SetCaretBlinkTime 582
SetCaretPos 583
SetClassLongA 584
SetClassLongW 585
SetClassWord 586
SetClipboardData 587
SetClipboardViewer 588
SetConsoleReserveKeys 589
SetCursor 590
SetCursorContents 591
SetCursorPos 592
SetDebugErrorLevel 593
SetDeskWallpaper 594
SetDlgItemInt 595
SetDlgItemTextA 596
SetDlgItemTextW 597
SetDoubleClickTime 598
SetFocus 599
SetForegroundWindow 600
SetInternalWindowPos 601
SetKeyboardState 602
SetLastErrorEx 603
SetLayeredWindowAttributes 604
SetLogonNotifyWindow 605
SetMenu 606
SetMenuContextHelpId 607
SetMenuDefaultItem 608
SetMenuInfo 609
SetMenuItemBitmaps 610
SetMenuItemInfoA 611
SetMenuItemInfoW 612
SetMessageExtraInfo 613
SetMessageQueue 614
SetParent 615
SetProcessDefaultLayout 616
SetProcessWindowStation 617
SetProgmanWindow 618
SetPropA 619
SetPropW 620
SetRect 621
SetRectEmpty 622
SetScrollInfo 623
SetScrollPos 624
SetScrollRange 625
SetShellWindow 626
SetShellWindowEx 627
SetSysColors 628
SetSysColorsTemp 629
SetSystemCursor 630
SetSystemMenu 631
SetSystemTimer 632
SetTaskmanWindow 633
SetThreadDesktop 634
SetTimer 635
SetUserObjectInformationA 636
SetUserObjectInformationW 637
SetUserObjectSecurity 638
SetWinEventHook 639
SetWindowContextHelpId 640
SetWindowLongA 641
SetWindowLongW 642
SetWindowPlacement 643
SetWindowPos 644
SetWindowRgn 645
SetWindowStationUser 646
SetWindowTextA 647
SetWindowTextW 648
SetWindowWord 649
SetWindowsHookA 650
SetWindowsHookExA 651
SetWindowsHookExW 652
SetWindowsHookW 653
ShowCaret 654
ShowCursor 655
ShowOwnedPopups 656
ShowScrollBar 657
ShowStartGlass 658
ShowWindow 659
ShowWindowAsync 660
SoftModalMessageBox 661
SubtractRect 662
SwapMouseButton 663
SwitchDesktop 664
SwitchToThisWindow 665
SystemParametersInfoA 666
SystemParametersInfoW 667
TabbedTextOutA 668
TabbedTextOutW 669
TileChildWindows 670
TileWindows 671
ToAscii 672
ToAsciiEx 673
ToUnicode 674
ToUnicodeEx 675
TrackMouseEvent 676
TrackPopupMenu 677
TrackPopupMenuEx 678
TranslateAccelerator 679
TranslateAcceleratorA 680
TranslateAcceleratorW 681
TranslateMDISysAccel 682
TranslateMessage 683
TranslateMessageEx 684
UnhookWinEvent 685
UnhookWindowsHook 686
UnhookWindowsHookEx 687
UnionRect 688
UnloadKeyboardLayout 689
UnlockWindowStation 690
UnpackDDElParam 691
UnregisterClassA 692
UnregisterClassW 693
UnregisterDeviceNotification 694
UnregisterHotKey 695
UnregisterMessagePumpHook 696
UnregisterUserApiHook 697
UpdateLayeredWindow 698
UpdatePerUserSystemParameters 699
UpdateWindow 700
User32InitializeImmEntryTable 701
UserClientDllInitialize 702
UserHandleGrantAccess 703
UserLpkPSMTextOut 704
UserLpkTabbedTextOut 705
UserRealizePalette 706
UserRegisterWowHandlers 707
VRipOutput 708
VTagOutput 709
ValidateRect 710
ValidateRgn 711
VkKeyScanA 712
VkKeyScanExA 713
VkKeyScanExW 714
VkKeyScanW 715
WCSToMBEx 716
WINNLSEnableIME 717
WINNLSGetEnableStatus 718
WINNLSGetIMEHotkey 719
WaitForInputIdle 720
WaitMessage 721
Win32PoolAllocationStats 722
WinHelpA 723
WinHelpW 724
WindowFromDC 725
WindowFromPoint 726
keybd_event 727
mouse_event 728
wsprintfA 729
wsprintfW 730
wvsprintfA 731
wvsprintfW 732