Must declare the scalar variable @variableName c# Sql

When u need to pass parameters to sql Query. This can be done in two ways..

First one:

SqlConnection connection = new SqlConnection(ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString);

string query = "select * from table where columnName = '" + txtSearch.Text + "'";

SqlCommandcommand = new SqlCommand(query,connection);

Bur this approach is not recommended. this may lead to error sometimes. So, whenever we need to pass parameters to query, use SqlParamater. The same query can be written as..

string query = "select * from table where columnName =@value";

SqlCommand command = new SqlCommand(query,connection);

command.Parameters.AddWithValue("@value",txtSearch.Text);

Using SqlParameters gives a cleaner, less error prone and SQL injection safe (comparative) code.

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LINQ Single and SingleOrDefault

Single means you are expecting only one result from LINQ query. If you get more than one or zero matches, then it throws exception.

SingleOrDefault means if you get single result, it returns that result but if it doesn't find any matches it returns default value but you will not get any exception.

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Return only one result from LINQ Query

You can do it in two ways, either use First or use Single.

First returns only first row, even there are multiple rows

Single expects only one row to be returned, and if there are multiple rows, it throws exception.

So, use Single if you are expecting only one row to be returned.

EX: var book= from book in store where (book.price > 100) select book).First();
Generally , the query returns all books having price greater than 100. but , as we are using  " .First() " only first book will be returned.

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