How to initialize the Expression.Call object in System.Linq.Expression C#

Question

I want to initialize the Expression Call outside the if else condition. As i have to use it to generate expression body because i have two different type coming from database i.e int and int?. My Code is below. I am getting error to instantiate the toString object.

 var parameterExp = Expression.Parameter(typeof(T), "type");
 var propertyExp = Expression.Property(parameterExp, propertyName);
 MethodInfo method = typeof(string).GetMethod(methodType, new[] { typeof(string) });
 var searchValue = Expression.Constant(propertyValue.ToLower(), typeof(string));

var toString = new MethodCallExpression();

if (propertyName == nameof(CustomerListDto.Id))
{
     toString = Expression.Call(propertyExp, typeof(int).GetMethod("ToString", System.Type.EmptyTypes));
}
else
{
    toString = Expression.Call(propertyExp, typeof(int?).GetMethod("ToString", System.Type.EmptyTypes));
}

var body = Expression.Call(toString, method, searchValue);
return Expression.Lambda<Func<T, bool>>(body, parameterExp);

I don't know exactly how to initialize ExpressionCall. This is the thing which i want to know. Currently it is giving me error "MethodCallExpression does not contains constructor that takes 0 argument". I searched a lot but could not find any solution.

"I am getting error" - what error? Please provide a minimal reproducible example with the complete results, whether that's a compile-time error or an exception. — Jon Skeet
– Jon Skeet, Commented Oct 3, 2017 at 9:36
@FilipCordas. No it contains value. It's datatype is Nullable Int. — Shahid Malik
– Shahid Malik, Commented Oct 3, 2017 at 9:40
You can't instantiate a MethodCallExpression manually as the constructor is private. You can get an instance of MethodCallExpression as the return value of Expression.Call though. You probably just want to declare it like "MethodCallExpression toString;" instead of "var toString = new()..." Then assign it with Expression.Call — Matt Searles
– Matt Searles, Commented Oct 4, 2017 at 4:57
@MattSearles. Thanks it helps. I have converted my code with ternary operator like this MethodCallExpression toString = propertyName == nameof(CustomerListDto.Id)? Expression.Call(propertyExp, typeof(int).GetMethod("ToString", System.Type.EmptyTypes)): Expression.Call(propertyExp, typeof(int?).GetMethod("ToString", System.Type.EmptyTypes)); Please give your solution in answer so i can mark it as accepted answer. — Shahid Malik
– Shahid Malik, Commented Oct 4, 2017 at 5:21

Matt Searles · Accepted Answer · 2017-10-05 05:20:20Z

1

You can't instantiate a MethodCallExpression manually as the constructor is private. You can get an instance of MethodCallExpression as the return value of Expression.Call though. You probably just want to declare it as

MethodCallExpression toString;

// Then assign it with Expression.Call(...);

if (propertyName == nameof(CustomerListDto.Id))
{
     toString = Expression.Call(propertyExp, typeof(int).GetMethod("ToString", System.Type.EmptyTypes));
}
else
{
    toString = Expression.Call(propertyExp, typeof(int?).GetMethod("ToString", System.Type.EmptyTypes));
}

answered Oct 5, 2017 at 5:20

Matt Searles

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