.NET Core Reflection Creating a Lambda Expression

Question

I am trying to create a lambda expression (using Reflection) that looks like

IServiceProvider provider => provider.GetService<TDbContext>()

Or, more specifically, as it GetServiceis an extension method:

provider => ServiceProviderServiceExtensions.GetService<TDbContext>(provider)

This is my code:

 var methodInfo = typeof(ServiceProviderServiceExtensions).
                GetTypeInfo().
                GetMethod("GetService").
                MakeGenericMethod(typeof(TDbContext));

        var lambdaExpression = Expression.Lambda(
            Expression.Call(methodInfo, Expression.Parameter(typeof(IServiceProvider), "provider")),
            Expression.Parameter(typeof(IServiceProvider), "provider")
            );

var compiledLambdaExpression = lambdaExpression.Compile();

I get this error

An exception of type "System.InvalidOperationException" occurred in System.Linq.Expressions.dll, but was not processed in the user code.

Additional information: the 'provider' variable of type 'System.IServiceProvider' refers from the scope '', but this is not defined

Answer 1

. Expression.Parameter , :

var methodInfo = typeof(ServiceProviderServiceExtensions).
            GetTypeInfo().
            GetMethod("GetService").
            MakeGenericMethod(typeof(TDbContext));

var providerParam = Expression.Parameter(typeof(IServiceProvider), "provider");

var lambdaExpression = Expression.Lambda(
        Expression.Call( methodInfo, providerParam ),
        providerParam
        );

var compiledLambdaExpression = lambdaExpression.Compile();