我正在尝试将编辑功能添加到用户提交的数据库记录中。问题是，混合中有一张图片（如字节）。该图片之前已被裁剪（主要是出于造型原因），当然，用户需要选择a）保留现有图片或b）选择并裁剪新图片。如果用户提交新图片，以下代码可以正常工作，但是如果没有新图片，它将覆盖现有图片而不是保留用户先前上传的图片。
这是我的控制器：
// POST: EditErrand
public ActionResult UpdateErrand([Bind(Exclude = "Picture")]EditErrandsViewModel model)
{
string newPic = Request.Form["editErrandCroppedPicture"];
byte[] imageBytes = Convert.FromBase64String(newPic);
var userId = User.Identity.GetUserId();
var errand = new EditErrandsViewModel
{
ID = model.ID,
UserID = User.Identity.GetUserId(),
FirstName = UserManager.FindById(userId).FirstName,
Email = UserManager.FindById(userId).Email,
Phone = UserManager.FindById(userId).PhoneNumber,
Hometown = UserManager.FindById(userId).Hometown,
Rating = model.Rating,
Category = model.Category,
SubCategory = model.SubCategory,
Title = model.Title,
Description = model.Description,
Location = model.Location,
ASAP = model.ASAP,
StartDateTime = model.StartDateTime,
DurationInHours = model.DurationInHours,
EndDateTime = model.EndDateTime,
DateTimePosted = DateTime.UtcNow.ToUniversalTime(),
Currency = model.Currency,
Offering = model.Offering,
Price = model.Price,
errandTax = model.errandTax,
PaymentMethod = model.PaymentMethod,
LatitudePosted = model.LatitudePosted,
LongitudePosted = model.LongitudePosted,
LocationPosted = model.LocationPosted,
UserActivityLatitude = model.LatitudePosted,
UserActivityLongitude = model.LongitudePosted,
UserActivityLocation = model.LocationPosted,
Published = false
};
if (imageBytes.Length > 2)
{
errand.Picture = imageBytes;
}
DB.Entry(errand).State = EntityState.Modified;
DB.SaveChanges();
var Success = new UserActivities
{
ActivityName = "EditErrand_Success",
UserID = User.Identity.GetUserId(),
ActivityTimeStamp = DateTime.Now.ToUniversalTime(),
ActivityLatitude = model.UserActivityLatitude,
ActivityLongitude = model.UserActivityLongitude,
ActivityLocation = model.UserActivityLocation
};
DB.UserActivityList.Add(Success);
DB.SaveChanges();
return RedirectToAction("errandPreview");
}
控制器中有以下语句，因为如果用户没有上传文件，它仍会将 0x 写入数据库。
if (imageBytes.Length > 2)
{
errand.Picture = imageBytes;
}
为了完整起见，我还添加了视图的相关部分，因此您可以了解如何完成裁剪（使用JCrop）：
<div id="editErrandPictureDisplay" class="errandomDisplay col-xs-offset-1 col-xs-10 col-sm-offset-1 col-sm-10 col-md-offset-4 col-md-7 col-lg-offset-5 col-lg-6">
<img id="editErrandPicture" class="editErrandPicture" src="@Url.Action("errandPicture", "errandom")" />
</div>
<div id="editErrandPictureArea" class="manageArea row">
@Html.LabelFor(m => m.Picture, new { @id = "editErrandPictureLabel", @class = "manageLabel col-xs-offset-1 col-xs-10 col-sm-offset-1 col-sm-10 col-md-offset-1 col-md-3 col-lg-offset-1 col-lg-4" })
<a id="editErrandPictureSelectionButton" class="manageField col-xs-offset-1 col-xs-10 col-sm-offset-1 col-sm-10 col-md-offset-0 col-md-7 col-lg-offset-0 col-lg-6" href="#">
select a file...
</a>
@Html.TextBoxFor(m => m.Picture, new { @id = "editErrandPictureField", @class = "manageField col-xs-offset-1 col-xs-10 col-sm-offset-1 col-sm-10 col-md-offset-0 col-md-7 col-lg-offset-0 col-lg-6", @type = "file", @style = "display: none" })
</div>
<hr />
<script>
jQuery("#editErrandPictureSelectionButton").click(function () {
$("#editErrandPictureField").click();
$("#editErrandCroppingArea").show();
});
</script>
<script>
$("#editErrandPictureField").change(function () {
var fullFileName = $("#editErrandPictureField").val()
$("#editErrandPictureSelectionButton").html(fullFileName.substr(fullFileName.lastIndexOf('\\') + 1));
});
</script>
<div id="editErrandCroppingArea" class="manageArea row" style="display: none">
<img id="editErrandOriginal" class="editErrandImage" src="" alt="" style="display: none" />
<canvas id="editErrandCropped" class="editErrandImage" height="5" width="5"></canvas>
<input id="editErrandButtonCrop" class="manageButton col-xs-offset-1 col-xs-10 col-sm-offset-1 col-sm-10 col-md-offset-1 col-md-10 col-lg-offset-1 col-lg-10" type="button" value="Crop" />
<input id="editErrandCropX" class="editErrandData" name="editErrandCropX" type="hidden" />
<input id="editErrandCropY" class="editErrandData" name="editErrandCropY" type="hidden" />
<input id="editErrandCropW" class="editErrandData" name="editErrandCropW" type="hidden" />
<input id="editErrandCropH" class="editErrandData" name="editErrandCropH" type="hidden" />
<input id="editErrandCroppedPicture" class="editErrandData" name="editErrandCroppedPicture" type="hidden" />
</div>
<script type="text/javascript">
$(function () {
if ($('#editErrandCroppingArea').width() > 500) {
$('#editErrandPictureField').change(function () {
$('#editErrandOriginal').hide();
var reader = new FileReader();
reader.onload = function (e) {
$('#editErrandOriginal').show();
$('#editErrandOriginal').attr("src", e.target.result);
$('#editErrandOriginal').Jcrop({
onChange: SetCoordinates,
onSelect: SetCoordinates,
aspectRatio: 1,
boxWidth: 450,
addClass: 'editErrandCropping'
});
}
reader.readAsDataURL($(this)[0].files[0]);
});
}
else {
$('#editErrandPictureField').change(function () {
$('#editErrandOriginal').hide();
var reader = new FileReader();
reader.onload = function (e) {
$('#editErrandOriginal').show();
$('#editErrandOriginal').attr("src", e.target.result);
$('#editErrandOriginal').Jcrop({
onChange: SetCoordinates,
onSelect: SetCoordinates,
aspectRatio: 1,
boxWidth: 250,
addClass: 'editErrandCropping'
});
}
reader.readAsDataURL($(this)[0].files[0]);
});
}
$('#editErrandButtonCrop').click(function () {
var x1 = $('#editErrandCropX').val();
var y1 = $('#editErrandCropY').val();
var height = $('#editErrandCropH').val();
var width = $('#editErrandCropW').val();
var canvas = $("#editErrandCropped")[0];
var context = canvas.getContext('2d');
var img = new Image();
img.onload = function () {
canvas.height = height;
canvas.width = width;
context.drawImage(img, x1, y1, width, height, 0, 0, width, height);
var image = canvas.toDataURL().replace(/^data:image\/[a-z]+;base64,/, "");
$('#editErrandCroppedPicture').val(image);
//$('#editErrandButtonUpload').show();
$('#editErrandCropped').hide();
$('#editErrandButtonCrop').hide();
};
img.src = $('#editErrandOriginal').attr("src");
});
});
function SetCoordinates(c) {
$('#editErrandCropX').val(c.x);
$('#editErrandCropY').val(c.y);
$('#editErrandCropW').val(c.w);
$('#editErrandCropH').val(c.h);
$('#editErrandButtonCrop').show();
};
</script>
所以最终我的问题是： 如果用户选择新图片（工作正常），如何更新图片？如果用户没有选择新图片，请确保不会覆盖以前上传的图片？现在，所有值将被更新，除了图片（除非选择并裁剪新图片）。
现在添加View Model后，我收到以下错误：
实体类型EditErrandViewModel不是当前上下文的模型的一部分。 描述：执行当前Web请求期间发生未处理的异常。请查看堆栈跟踪以获取有关错误及其源自代码的位置的更多信息。
异常详细信息：
System.InvalidOperationException: The entity type EditErrandViewModel is not part of the model for the current context.
来源错误：
Line 348: errand.Picture = imageBytes;
Line 349: }
Line 350: DB.Entry(errand).State = EntityState.Modified;
Line 351: DB.SaveChanges();
Line 352: var Success = new UserActivities