System.Text.Json parse document that exists internal to a string - json

Question

I receive string content that starts with a JSON value (could be simple or complex) and has some additional content afterward. I'd like to be able to parse the JSON document.
I don't have control over the string, so I can't put any kind of delimiter after the JSON content that would enable me to isolate it.
Examples:
"true and some more" - yields <true>
"false this is different" - yields <false>
"5.6/7" - yields <5.6>
"\"a string\""; then this" - yields <"a string">
"[null, true]; and some more" - yields <[null, true]>
"{\"key\": \"value\"}, then the end" - yields <{"key": "value"}>
The issue is the trailing content. The parser expects the input to end and throws an exception:
')' is invalid after a single JSON value. Expected end of data.
There isn't an option in JsonDocumentOptions to allow trailing content.
As a bonus, if you can give a solution that uses ReadOnlySpan<char>, that'd be aweseme.

Answer 1

The suggested answer of the custom reader wasn't working for me because the problem existed in the base reader: it just doesn't like certain trailing characters.
Since I still wanted to rely on JsonDocument.Parse() to extract the element for me, I really just needed to find where the element stopped break that bit off as a separate piece and submit that to the parse method. Here's what I came up with:
public static bool TryParseJsonElement(this ReadOnlySpan<char> span, ref int i, out JsonElement element)
{
try
{
int end = i;
char endChar;
switch (span[i])
{
case 'f':
end += 5;
break;
case 't':
case 'n':
end += 4;
break;
case '.': case '-': case '0':
case '1': case '2': case '3':
case '4': case '5': case '6':
case '7': case '8': case '9':
end = i;
var allowDash = false;
while (end < span.Length && (span[end].In('0'..'9') ||
span[end].In('e', '.', '-')))
{
if (!allowDash && span[end] == '-') break;
allowDash = span[end] == 'e';
end++;
}
break;
case '\'':
case '"':
end = i + 1;
endChar = span[i];
while (end < span.Length && span[end] != endChar)
{
if (span[end] == '\\')
{
end++;
if (end >= span.Length) break;
}
end++;
}
end++;
break;
case '{':
case '[':
end = i + 1;
endChar = span[i] == '{' ? '}' : ']';
var inString = false;
while (end < span.Length)
{
var escaped = false;
if (span[end] == '\\')
{
escaped = true;
end++;
if (end >= span.Length) break;
}
if (!escaped && span[end] == '"')
{
inString = !inString;
}
else if (!inString && span[end] == endChar) break;
end++;
}
end++;
break;
default:
element = default;
return false;
}
var block = span[i..end];
if (block[0] == '\'' && block[^1] == '\'')
block = $"\"{block[1..^1].ToString()}\"".AsSpan();
element = JsonDocument.Parse(block.ToString()).RootElement;
i = end;
return true;
}
catch
{
element = default;
return false;
}
}
It doesn't care so much about what's in the middle except (for strings, objects, and arrays) to know whether it's in the middle of a string (where it would be valid for the end character to be found) and checking for \-delimited characters. It works well enough for my purposes.
It takes a ReadOnlySpan<char> and an integer by reference. i needs to be the start of the expected JSON value, and it will be advanced to the next character after, if a valid value is found. It also follows the standard Try* pattern of returning a bool with an output parameter for the value.