实体框架 EF.Functions.Like 与 string.Contains

Question

I was reading the announcement of entity framework core 2.0 https://blogs.msdn.microsoft.com/dotnet/2017/08/14/announcing-entity-framework-core-2-0/

It says that they added new Sql functions like EF.Functions.Like for performing the SQL LIKE operation.

I was wondering, what then would be the difference between EF.Functions.Like and string.Contains / StartsWith ?

For example:

var customers = context.Customers.Where(c => c.Name.StartsWith("a")); // Version A
var customers = context.Customers.Where(c => EF.Functions.Like(c.Name, "a%")); // Version B

What would be the difference between the two versions? EF already knows how to translate string.Contains / StartsWith to the corresponding SQL operations, doesn't it?

The only reason i can think of is that EF.Functions.Like would allow for more complex patterns like "a%b%" (although this one can be written as StartsWith("a") && Contains("b") )

Is this the reason?

Answer 1

The answer of @adiga is quite incomplete and covers just a part of the differences in usage.

However, .StartsWith(...) , .Contains(...) and .EndsWith(...) are also translated differently into SQL then EF.Functions.Like .

For example .StartsWith gets translated as (string LIKE pattern + "%" AND CHARINDEX(pattern, string) = 1) OR pattern = '' where .Contains gets translated into (CHARINDEX(pattern, string) > 0) OR pattern = '' .

EF.Functions.Like however gets translated into string LIKE pattern [ESCAPE escapeChar] .

This may also have implications on Performance. The above is valid for EF Core SqlServer provider. Other EF Core providers may translate it differently.

Answer 2

Like query supports wildcard characters and hence very useful compared to the string extension methods in some scenarios.

For ex: If we were to search all the 4 lettered names with 'ri' as the middle characters we could do EF.Functions.Like(c.Name, "_ri_");

or to get all the customers from cities which start with vowels:

var customers = from c in context.Customers 
                   where EF.Functions.Like(c.City, "[aeiou]%")
                   select c;

(Please read @Tseng's answer on how they are translated differently into SQL queries)

Answer 3

@adiga is quite incomplete and covers just a part of the differences in usage.

However, .StartsWith(...) , .Contains(...) and .EndsWith(...) are also translated differently into SQL then EF.Functions.Like .

For example .StartsWith gets translated as (string LIKE pattern + "%" AND CHARINDEX(pattern, string) = 1) OR pattern = '' where .Contains gets translated into (CHARINDEX(pattern, string) > 0) OR pattern = '' .

EF.Functions.Like however gets translated into string LIKE pattern [ESCAPE escapeChar] .

This may also have implications on Performance. The above is valid for EF Core SqlServer provider. Other EF Core providers may translate it differently.