c# - TrimStart in Expression Tree with LINQ to Entities -

i'm attempting write expression tree can allow dynamic use of startswith() method on non-string valued columns using entity framework.

e.g. integervaluedcolumn.startswith(5) return 500, 5000, 555, 5123, etc

i trying write expression tree based on answer:

how query integer column "starts with" in entity framework?

here have far:

methodinfo stringconvert = typeof(sqlfunctions).getmethod("stringconvert", new[] { typeof(double?) });  expression castexpression = expression.convert(propertyexpression, typeof(double?));  expression convertexpression = expression.call(null, stringconvert, castexpression);  methodinfo trimstart = typeof(string).getmethod("trimstart");  expression nullexpression = expression.constant(null, typeof(char[])); expression trimexpression = expression.call(convertexpression, trimstart, nullexpression);  methodinfo startswith = typeof(string).getmethod("startswith", new[] { typeof(string) });  expression methodexpression = expression.call(trimexpression, startswith, constantexpression);  return methodexpression; 

when compile , run expression, following exception:

the method 'system.string trimstart(char[])' supported in linq entities when there no trim characters specified arguments.

in original example, expression is:

sqlfunctions.stringconvert((double)x.accountnumber)     .trimstart().startswith(searchterm) 

but comes out as:

stringconvert(convert(x.accountnumber)).trimstart(null).startswith(searchterm) 

i have removed 2 lines dealing trimstart (nullexpression , trimexpression) , verified statement runs (excluding thought error being caused different language use). theory based on exception message trimstart() method wants called 0 parameters, when try expression builder tells me incorrect number of parameters have been passed in figure i'm missing something.

how go calling trimstart method trimstart() instead of trimstart(null) or trimstart(new char[0]) using expression trees?