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If you find any instances of plagiarism from the community, please send an email to: info-contact alibabacloud. A comprehensive suite of global cloud computing services to power your business. The difference between null, "", Dbnull,string. Tags dotnet. Related Keywords: difference between null and not null in sql difference between variable and constant in c difference between structure and class in c difference between continue and break in c difference between break and continue in c difference between string and character in c difference between while and while in c.
If there is a first row, but the first column is empty, then the return is DBNull. If the first row of the first column is not empty, then the executescalar directly corresponds to the value of the dotnet.
This is the rule. Unless you think that CMD executes, there must be at least one row of data, otherwise there will be an error. ExecuteScalar ; Will still go wrong, because when the above statement is not true, no rows are returned. Value, if NULL, indicates that the parameter is not specified or represents default. If DBNull. Value, represents null in SQL. You can use Convert.
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Value , is a valid reference to an instance of System. DbNull System. DbNull is a singleton and System. So, there's a big conceptual difference between the two. The keyword null represents an invalid reference.
The class System. DbNull represents a nonexistent value in a database field. In general, we should try avoid using the same thing in this case null to represent two very different concepts in this case an invalid reference versus a nonexistent value in a database field.
Keep in mind, this is why a lot of people advocate using the null object pattern in general, which is exactly what System. DbNull is an example of.
In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:. It can be assigned to a variable as a representation of no value:.
From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself undefined while null is an object.
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