Base-64字符串无效，The input is not a valid Base-64 string as it contains a non-base 64 character

base64规则：

* 字符串只可能包含A-Z，a-z，0-9，+，/，=字符
* 字符串长度是4的倍数
* =只会出现在字符串最后，可能没有或者一个等号或者两个等号

首先，C# 做上传文件的时候，需要替换掉这些字符：

data:image/jpeg;base64,

data:image/png;base64,

data:image/gif;base64,

if (str.Contains("base64,"))
{
  str = str.Split("base64,")[1];
}

如果还存在错误的话，可以进行一些字符替换试试：

var imgData = "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";
//过滤特殊字符即可    
string dummyData = imgData.Trim().Replace("%", "").Replace(",", "").Replace(" ", "+");byte[] byteArray = Convert.FromBase64String(dummyData);

上面两步操作完了还没有解决问题的话，可以调试看看base64字符串的长度，如果长度不是4的倍数，比如8057或者8041，可以尝试截掉最后一个字符，使字符长度变为4的倍数。(能正常解决bug，但不确定是否会产生其他问题)

str = str.Substring(0, str.Length - 1);

上面两个长度其实是本人调试的时候遇到的，神奇的是，复制粘贴后就会变成8056长度，可以正常解析。如果有大佬知道是怎么回事的话，欢迎评论沟通。