Value="<%#Eval("ColumnName")%>"

The opening quotes closes after (". So compiler can parse till Value="<%#Eval(" and would expect it to be terminated there. The rest of the statement i.e. ColumnName")%>" is meaningless and the compiler can't parse it, obviously resulting in an error. Finally Value="<%#Eval(" can't make it data-bind.

The syntax highlighter in the question's code shows the problem perfectly: if you use double quotes inside an XML attribute (and that is excactly what the Value= inside the asp.HiddenField is) after starting the attribute value with a double quote, this effectively ends the string literal. XML allows you to use either single quotes (') or double quotes (") to delimit an attribute's value, but the quotes have to match. XML always takes the first matching quote to end the corresponding starting quote. So if you want to use double quotes inside the attribute value (because C# needs them), you have to use single quotes to delimit it. This code works:

<asp:HiddenField runat="server" Value='<% #Eval("ColumnName") %>' />

for the content of the <% … %> part is interpreted in .net, the quotes inside the attribute value have to be double quotes. For a programming language that accepts strings delimited with single quotes, the following would work, too:

<asp:HiddenField runat="server" Value="<% #Eval('ColumnName') %>" />

I believe it's because when the compiler parses the expression, it actually sees two strings: <%#Eval( and )%>, and columnName is between those elements. That's why it says that the tag is not well formed. But if you use single quotes , everything is ok. Now, let's try to change the code and use something like Value="<%#Eval('ColumnName')%>". This time it doesn't work, we get an exception: Too many characters in character literal. We got an exception becuse single quotes are used to denote a char in .net. ColumnName is more than one char so we get an exception. In my opinion, the solution that starts with a single quote works, because inside those single quotes we do not have a normal string, but the expression that is evaluated on the server.

test editor

XMl attribute always checks for open double quote to end so compiler cannot understand Value="<%#Eval(" for parsing. Since it has no meaning it will simply terminate and gives error as "Server tag not well formed ". XML always takes the first matching quote to end the corresponding starting quote. If we want to use double quotes inside the attribute value, we have to use single quotes to delimit it

 Value='<% #Eval("ColumnName") %>' />

 Value="<% #Eval('ColumnName') %>" />

Either of above 2 are correct.

The error is shown because when parsing Value="<%#Eval("ColumnName")%>" HTML parser sees two strings. The first one is "<%#Eval(" and the second one is ")%>". So, the parser will assume that "<%#Eval(" is a string for Value and would not be able to parse the rest showing "The server tag is not well formed" error. However if you use Value = '<%#Eval("ColumnName")%>' HTML parser will consider '<%#Eval("ColumnName")%>' as one string and then it will be able to see "ColumnName" inside of it.

Please note that another solution Value="<% #Eval('ColumnName') %>" (as Vamshi suggested) will not work neither in C# ("Too many characters in character literal") nor in VB ("Expression expected").